any time. there is no "skill" standard to start playing for bag tags. tags are a fun way to initiate some competition between you and fellow golfers, particularly if you don't feel ready for leagues or more organized golf.
just be aware, if you play in a league event (league/tournament/etc) and someone else on your card has a tag, you're supposed to be playing for them. no hoarding a decent tag number because you don't want to lose it.