Currently both me and my husband have been helping my baby girl to change diapers and bathing her. How old should my husband stop to change her diaper and bath her.

I don't think there is an age that a dad should stop changing his daughter's diaper or stop bathing her. Only once she becomes old enough to bath by herself, at which point she'll presumably be out of diapers for a long time already. Same as if you had a Son. No reason why you shouldn't continue to change his diaper for as long as he's in them.