Can I still attend an aa meeting if I'm under 21?

Yes! There is no minimal age, although some very young adults or teens may find it uncomfortable with so many adults in the room. That being said, i would commend you on any decision to attend a meeting and encourage you to go first, i suspect many of the attendees would welcome you and even help steer you toward other meetings with younger members.
Absolutely. All are welcome but look for a meeting with other young people.