This is a fairly simple question but may have wider-reaching answers: when is it appropriate to validate entry in a wheel-based date picker (such as the iOS standard ones)?
Since there are three wheels for day, month, and year, and since a user could enter those three pieces of information in any order, there's no real way to ascertain when they're finished. Also, the three fields must have a default state, which we set to today's date.
We currently validate on the "value changed" event of the picker. So, the default state of the picker as they see it may be 31/12/2014, the user may scroll the day field first, and at that point it assesses, say, 10/12/2014, which is not allowed (as the user must be over 13 years old).
The basic issue is that we can't tell when the user has completed their entry, since they could enter day, month, then year, or year, month, then day, or perhaps the month was correct already and they didn't need to touch it, etc.
So, when would the user expect the validation (and any subsequent error message) to occur, and when is it safest to check the status of the picker?