long in the tooth

long in the tooth (to do sth) (old or getting old; and by extension past one's prime; over the hill — a bit too old to do something) — не первой молодости, в годах; староват уже (для чего-л.)

Example: Don't you think she's a bit long in the tooth to be a romantic heroine?(СDAmId)

see also
[no spring chicken]
[over the hill]
[past one's prime]

------
Originally about horses and, figuratively, about people