My latest is a 32GB
However, there are USB drives and USB drives. You can find some crappy no-name stuff that will never hold even half of the announced capacity, and some crappy fake brand stuff as well, so the store where you buy things is important. Good brands: Sandisk, Lexar, Kingston, Verbatim. Some deals are too good to be true.
Furthermore, given the capacities, support for USB3 really makes a difference: you go from 20MB/s to 90-100MB/s, so while filling a 32GB disk at 100MB/s is still 5 solid minutes, it is still vastly better than the half-hour required with USB2.
Anything under 8GB is ridiculous these days. Under 8GB, find leftover camera SD cards and use a card reader (most laptops have a built-in one)
The "sweet spot" size (best size/price ratio) is the 16-32GB range anyway.
Around age 13, the ruggedness is important, as well as a cap that's hard to lose. But it should still be usable in a USB port that is a bit recessed.
The cheap ones aren't very good backup devices IMHO because their reliability is crap.
PS: in my family of 4 I tallied 24 USB keys, from 128MB to 32GB, and around 50 SD cards, from 64MB up.