Teenbaby or TB is a term used in the Adult Baby (or Infantilist) community to describe teens (13-20) who like acting like babies. It's a paraphilic fetish. Being a TB usually involves;

Wearing, wetting, and dirtying, diapers.

Wanting to be treated like a baby. This can, in some cases, be a truly overpowering desire to regress to being a baby all the time. In most it manifests as roleplaying with a partner (sexual or otherwise, often also a TB).

Having, and playing with, objects associated with infancy. These include, but are not limited to, pacifiers (dummies), bottles, oversized baby clothes (romper suits, plastic pants, onesies etc.).

Being a TB can be a nightmare (I know, I'm one), for several reasons;

It's rare. Estimates range from 1:1000, to 1:20000. This is much rarer than being gay, bi, into BDSM, or being a furry.

Around 1-3% of TBs are female. 85% of male TBs are straight. This obviously creates ridiculous competition for the few girls in the TB community.

The TB/AB community is not very large, and as such not very visible. My furry friends have much less problem hooking up with other furries, than I do with other TBs; and he has fewer problems when he tells people he's a furry.

However, being TB can be a lot of fun, and besides, most people who try to supress it fail.