To add, "shall" here is exceedingly formal to the point of being incorrect in modern usage, at least in my (American) opinion. If OP's app says "shall" is the correct answer I would hesitate to trust it on anything else.

@randomhead - to me (I shall be 70 next April), UK (English), middle class, using 'shall' is a little old-fashioned and rather formal but not howlingly archaic and definitely not 'wrong' or 'incorrect'. Cambridge Dictionary quoting English Grammar Today says 'Shall is only used for future time reference with I and we, and is more formal than will'. In July I shall be 50 (The Times, 2018), in another six months, I shall be 50, and, therefore, become entitled to benefit (Hansard - UK Parliamament record, 1964)

'I shall be 50 on 31 October [2016]' The Spectator

I might very well say to my wife 'If the train is late, I shan't be pleased', and no strangeness would be perceived.

"Charles, I shall be off tomorrow; I almost envy you the broken arm which keeps you here.” - La Vendée (Anthony Trollope, 1850). I do love Trollope.

A Concise Grammar of Contemporary English by Randolph Quirk and Sidney Greenbaum (1973 - New York, Harcourt Brace Jovanovich) says: "Will for future can be used in all persons throughout the English-speaking world, whereas shall (for 1st person) is largely restricted in this usage to southern British English." I wouldn't call a documented usage from 1973 madly archaic).

I think the formality of “shall” might be a difference between BrE and AmE. In the USA, I almost never hear “shall” used in any context other than legal - certainly not in everyday spoken English. If someone said “I shan’t be pleased” to me, I’d ask them what that quote is from, because I’d never think that’s just someone talking.

Do you perceive “I am sixty tomorrow” and “I am turning sixty tomorrow” as equally idiomatic? My impression (as a non-native speaker) is that the latter is perfectly fine because it's referring to a specific point in the future, whereas the former is odd, I think because tomorrow is only the start of the period during which “be sixty” is true.

@Gilles'SO-stopbeingevil' - One can correctly say 'I am sixty tomorrow' and be understood by any native speaker to mean 'I shall reach of the age of sixty tomorrow', because we use the present-in-the future about scheduled events, including birthdays. No other interpretation makes sense, and indeed we often use this form.

@MichaelHarvey Sure, but my question isn't whether it would be understood, it's whether it sounds idiomatic or weird.

@Gilles'SO-stopbeingevil' - OK. Here's the answer, which I had thought I had already supplied: "The former" (I am sixty tomorrow) is idiomatic, and is not "weird".

@Gilles'SO-stopbeingevil' - to clarify - the present or past tense of 'to be' used with an age, and a time reference, e.g. I am 60 tomorrow/next week/next month/next year, or I was 60 yesterday/last week/last month/last year is invariably understood by native speakers to mean 'I reach or reached that age in that time frame'. Any other meaning would be rejected by the 'common sense' part of our brains as nonsensical, and because of our knowledge of the idiom.

@MichaelHarvey In English technical writing, it is an international standard that "shall" signiifes a requirement - i.e. something MUST occur something or MUST be done. "Will" signifies a statement of fact. Given the increasing prevalence of the technical use of English, "I shall be 20 years old next month" now sounds odd - it implies that the speaker has to do something in order to become 20 years old next month! "I shall be 20 years old next month" is as unidiomatic as "I must become 20 years old next month" IMO.

@Gilles'SO-stopbeingevil' - is it clear now?

I agree that, in some colloquial situations, it can be used this way, but it would still be seen as non-standard. I think this sort of distinction is useful to make for ESL speakers, as non-standard English from an ESL speaker is more likely to be perceived as a mistake, rather than a colloquialism. It's best to know what the standard is, and then know that you can deviate from it. I would prefer this answer make it clear that "Ishall" or "will" would be more standard, as would be "going to be" or "I'll be".

My experience as a native speaker of AmE with 8 years of experience teaching ESL is that present simple is used for future events, but it is not used about future states. "I am 60" refers to a state, not an action or event, and thus should not be used for the future. Perhaps usage is different in some parts of BrE.

@trlkly - I am 60 next week, my train leaves at 9 AM tomorrow, my daughter graduates next Tuesday, we fly to Madrid next month are not 'colloquial' in British English.

@alephzero: which international standard is that?

@psmears There is RFC 2119 which defines "must"/"shall", "should" and "may"

@user49822: Right - I'm aware of that - but that's more of a "convention used when writing internet-related specifications" than an international standard...

@Michael Harvey - Also in BrE, all of your extra examples sound fine to me, but the first one sounds a bit odd. It's unambiguous, but it doesn't sound natural. I definitely don't agree that it's idiomatic - if I were to read it, without an accent for context, I would assume a non-native writer. I think it's because "am" is the wrong sort of verb to use in that context, since it doesn't describe an event or an action, but a state. In contrast, "I'm turning 60 next week" sounds perfectly natural (as does "I'll be 60 next week", of course).

@Nye - I stand by my assertion that my examples are all 100% idiomatic for British English. What you mean by the 'first one'? Which one?

@Michael Harvey - "I am 60 next week". Perhaps this is very regional? (Sorry I was thinking of the examples in the comment, not the answer, when I said "first one")

@Nye Normal in British English which is, effectively 'regional' to a large part of the world. "Wrexham MP Ian Lucas will stand down at next election - 11 Oct 2019 — "I am 60 next year. I have been an MP since I was 40. I think the time is right for me to choose to follow a different path in the years to come" (BBC) - Ex-soldier from Hornchurch in diving world record bid: 14 Feb 2016 — He said: “I am 50 next year and so this is my last chance to get the record. I want to stay underwater for five days." Hornchurch Recorder (Essex newspaper).

Okay, if you think this is "regional" to - in effect - "the whole of the UK" I'm now confident enough to vote this down on the grounds that it's simply incorrect as that position is not defensible. That is, as soon as you have multiple people saying that they disagree, then it can't possibly be ubiquitous.

@Nye - many people may disagree with the truth. That does not make it false. I gave quotes from Wales and Essex - what's wrong with those?

Dear Mods - there are far too many comments here.

There's definitely an AmE "sounds fine but maybe a bit odd if I think about it; might mention it (but not take red pen to it) if editing a close friend's paper" and BrE "this is an absolutely bog-standard construction; there is nothing odd about it whatsoever" gulf between commenters here.

@neph - it's interesting that my answer has attracted both upvotes and angry comments copiously. I reckon my quote from and picture of CH Spurgeon helped a bit (appealed to the Christian voters).

You can use the verb "to be" when talking about a scheduled event in the future, such as a birthday, but being a particular age is not a scheduled event. That's why "it's my birthday tomorrow" is correct, but "I'm 20 tomorrow" is not.

@AaronF - 'I'm 20 tomorrow' (meaning it's my 20th birthday tomorrow) is 'correct' and not in any way strange, odd, or informal, in British English.

I really don't understand why the mods haven't pruned these comments. Maybe I'll raise it in Meta.

It's 'correct' as in it's used colloquially and understood, but grammatically it's not correct because "20" is an adjective, and follows the standard rules of using the verb 'to be' + adjective: "I am 20", "I will be 21 next month", "I was 19 last year". Other colloquialisms are frequently used and understood: "you crazy!", for example, is commonly understood to mean "you are crazy!", but that doesn't make it grammatically correct. (That probably isn't the best example, but it's the first one that popped into my head :-) )

re. the comments - you can flag your own answer and ask a moderator for action

Whenever you have seen comments pruned, at least in the last two years, it's most probably because a user or the OP themself has flagged it for the mods' attention. You can also choose not to respond/defend/argue that tends to do the trick. Not every comment is a valid observation, not every native speaker knows what they are talking about.

Also note that the question hit the HNQ (Hot Network Questions) which means it attracted a lot of visitors who don't normally participate on ELL or appreciate that the English language is super flexible and what is considered standard in one dialect is not in another.