it varies, and even the reasons for taking is different among each individual. while pills are for treating ED, ppl also uses pills for reasons like prolonging sessions, firing more rounds within a shorter time etc. so there's no real age to start, just take it when u feel u need it
I never thought of using pills before with my previous exs, until I meet my new gf. she's very horny lol so now im getting some so that our sessions can be more fun