How old it is may not even correlate that well with how long it receives updates. There are some models/revisions that are available for a few years, but the earliest ones in the series may receive updates as long as the last ones in the series.
I bought an Acer Chromebook in 2022 just because it was cheap and can do a bunch of things well enough that I don't need a more powerful machine. That being said, I've charged it inconsistently and perhaps the battery life has been pretty poor as a result. I looked up the stats and it's showing 87% battery after less than 20 cycles. But I don't use it that much on battery, so that may not be that much of an issue. There are replacement batteries available, but some of them seem way too much considering I bought it for $100 new at Target (but on sale). I still can't figure out how it was that cheap given it's got a decent (Lite-On) 45W USB-C power supply with a 11.6" display, processor, case, etc.
I won a used Dell Chromebook in a drawing back in 2019. Kind of sad (dim portion of the backlighting) but it worked. However, it stopped receiving updates around 2021 I think and finally failed around 2022.