Where does it say this? The only mentions of “age” in the ToU are “features may depend on your age”, “provide an accurate age”, a bunch of “agents”, and a California specific law. The rules about “age” in the Community Rules all seem to refer to actual IRL age as well. (It’s all stuff like Agencies, don’t ask for PII including age, users of all ages are on platform, no discrimination based on age, <13s can’t view links.)
Ignoring whether it’s allowed or not, hiding it won’t actually do anything to prevent exploiters. Plenty of them use really old PGed accounts, and most exploiters I’ve seen use at least 2019 accounts, if not older.