I started playing Blackjack in Reno and Tahoe when I was just 15. My sister and I would go up for a weekend 3 or 4 times a year when she was 18 and I was 15. We would play sometimes for 8 or 9 hours straight on just $100 each (this was back when $1 tables and single deck blackjack could still be found). We often became friendly with the dealers and, sometimes, even the pit bosses. Never had a problem with being asked for ID. We even were served the free alcohol at the tables, although we ordered it more to look "grown up" than to really drink since neither of us like the taste of alcohol. I think the trick is to act confident and not give away by your attitude that you are doing anything wrong.
My nephew was able to successfully play on a Princess ship last year at age 17. He doesn't look particularly old for his age, so I don't think he would have gotten away with it in a land-based casino where they have tighter regulations and enforcement spies to worry about.