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I know there is no addon for this but is there any trick that can do this? For SMF there is a mod

it's useful to me to understand the target age of my users.

Core features / Advanced Profile Fields have settings that make a field (1) a DATE type and (2) mandatory on registration. Do you think these could help? (visibility and edit access can be set, too)

I didn't check, but I think hugbear is right. And if it doesn't work... it's a bug!

I'll check but it will not act like the birthday field.
I think there was some discussions to make the birthday and location act like custom field

Ohhh... you are right: birth date is the only "custom field" that doesn't act like a custom field.