Let's say I knew how old somebody is at the moment, but I forgot it. In that case, I think I can ask, "How old were you?", am I right?
Sometimes the past tense is used in a question when the speaker is unsure of information given earlier in the conversation, perhaps because of a faulty memory, or would like the other person to repeat it. Sometimes the past tense referring to an earlier moment in the conversation is part of a conversational ploy, or an expression of surprise.
How old were you again? Did you say 23? You would have been at the school when my cousin, Jane, was attending. Did you ever cross paths with a Jane Smith when you were there?
It doesn't have to be a full-blown conversation. It can be a brief transaction:
How much was that? Did you say $11.45? That's rather pricey for a latte!
P.S. The past tense, along with optional signals like again, implies that the answer to the question was previously known by the speaker. That implication could be true, the speaker might actually have known it once, or it could be just a conversational gambit.
P.P.S. We can use the past tense when asking about almost anything that we have already been told by the person we're speaking with (or even by their mom, dad, brother, sister, cousin, best friend, etc).
Where was your brother going to school again?
The past tense there is an indicator that we have already been told this information, either by the person themselves or by someone close to them with whom we are on speaking terms.
The thing asked about may be in the present or even in the future:
Where was it you were going next week?
The past tense simply indicates that the fact was already told to the speaker, what that is true or not. It is presented as something the speaker cannot remember at the moment.
Looking at your chat with Tᴚoɯɐuo, the context is clear to me: there is no conversation between some famous personality and you. It's just that you had read somewhere or you knew his age and when you go and start talking to him, at that very moment, you don't remember his age. And, you are asking whether "How old were you?" is okay?
NO, in this context, it's certainly not.
Because the celebrity in that case has no clue what you are asking! Probably, he would wait for a question to get completed...
How old were you (when you first played against Australia)? :)
Now, if the conversation was going on and the celebrity mentioned his age but after a few minutes you forgot the number, you may then ask...
How old are you again?
That again shows that recently, the information was revealed but you don't recall it now. I may not prefer putting was to avoid confusion. Because in this context, 'is' should work.