I’m running Gentoo on a 15-year-old laptop (HP Pavilion from 2008, Athlonx2 1.7GHz, 2GB RAM, 100GB HDD), but that’s only practical if you’re willing to let it compile things overnight. Anyway, based on my experience, you’ll get a workable machine out of it, but large programs like modern web browsers will have a certain amount of startup lag.

If your laptop has a 64-bit CPU, any standard distro will probably work. If it’s a 32-bit CPU, you’ll need a distro that still supports that, of which Debian is probably the most popular (other options would be openSUSE, Mageia, and, yes, Gentoo). If this laptop was a lower-end machine 15 years ago, you may want to consider Puppy Linux.

You’ll want a lighter or older desktop environment (MATE, TDE, XFCE, LXDE, maybe Lumina) or a plain window manager, as others have said—do not try to install current Gnome or KDE (so the main Ubuntu flavour is not a good choice).