Hai, huge 3DS gamer here.
Uh, what? Too old? Most Nintendo events I attend have more thirtysomethings than kids or young adults, haha. You're never too old to enjoy video games and you shouldn't be ashamed.
The 3DS has an amazing game library now, and lots of the great games are supercheap because the system's been out a while. The streetpass feature and its related activities are really awesome, too - take your DS with you in your purse, "pass" other people and get bonuses in some games, plus play really neat mini-games that are built into the system.
I love that I can download full games into my system as opposed to having a million cartridges. I have Pokemon, Animal Crossing, Harvest Moon AND Ace Attorney all at my fingertips on the go, without having to carry (and risk losing) cartridges. The e-shop also has retro games and other fun stuff available.
I have a 3DS XL. Sadly it's pink, but I skinned it.
I love it. I never use the 3D. Half the games I have don't really make use of the 3D either, haha. It was a limited-novelty thing and you don't need the 3D on, ever. The 3DS has way better graphics capability than the original DS, so it's still worth the dosh.
I personally would never go with a 2DS, I tried one in person at the Pokemon X/Y launch party and the screens are even smaller than the regular 3DS. ugh, pass. Nintendo designed it specifically for the younger (age 6-12) age bracket for less chance of it breaking and no fear of the 3D hurting a child's eyes. I like the XL screens, much kinder on my eyes.
If you want to know anything else, let me know! The BF and I are huge gamers, especially with 3DS. We're members of the local Streetpass/3DS game group too. Nerds are we.
ETA: For good geeky measure, here's some pics of my skinned 3DS XL. I get a lot of comments on the custom skin.
It's "Volcano" by Alex Noriega. Just the right amount of girly without being too... pink.
And the crocheted cozy I made for it so it wouldn't get all scuffed up in my bag while I collect Streetpasses!