Understanding what you want it to mean is important. I got my first tattoo at age 22, but I don't consider that being the best age necessarily. The key to not regretting it, like Scylian said, is to think it through. The people I've known that regretted it either had names or symbols that either meant something to them that changed, like the name of a now-ex girlfriend or a logo of a company that they don't really like anymore, and it seems stupid to them. Or they break a few rules of thumb I'm aware of:
1: No text. Just save yourself the trouble, don't get text. Even if it doesn't fall prey to the ex-girlfriend or warping problems, or typos, it's often something that may not carry on for more than a few years. Obvious exceptions are important dates (known a few veterans that got birthdates or dates of deaths of battle brothers) or names of your offspring.
2: No advertising. Seriously, if you want to give a company advertising space on your body, buy a tshirt or a hat. I've known too many people who got anything from Nike to MLB to restaurant logos. No they didn't get paid for it, they just really loved those things at the time.
3: Don't be edgy with it. If you're gonna try to piss people off with art on your skin, it will eventually work, and it will not turn out well. Best case scenario is that the only person you really piss off is yourself, and tattoo removals are rarely cheap.
4: Be sober, be awake, be thoughtful, and take your time. If you get an idea for a tattoo that you really want, sleep on it, probably for a good while. Before I got the tattoo I have, I'd wanted that exact design for over a year, though I think that's overkill. Just make sure it's not something you're obsessed with now, like a band or a show, but won't care about in a few months/years.
I've luckily never regretted mine, though I do kind of regret getting one and then never having the money to get another. The process is addicting, in my personal opinion. I totally understand how someone can start with something small and end up with a full sleeve or more in a few years' time.