Can I race in an older age group or in adult races?

Usually, yes:

• Juniors can race in their own age group or any older age group, as long as they meet that race's category requirements.
• Juniors cannot enter Masters races.
• At state, regional and national championships, you can only race in the events for your own racing age and/or category.
• In mountain bike endurance races, juniors 14 and under can race up with older juniors at the organizer's discretion, but should generally enter races for riders up to Category 2.
• Girls can enter boys' races they're eligible for by age, category and any performance standards, plus categorized boys' races up to one category below their own (up to two below for Category 1 women in road, track and cyclocross). This doesn't apply at national championships.

If you race up on the track, you can use that race's gear limit.