As long as we're operating purely off of opinion and feel and no actual numbers because I don't feel like perusing the wiki for an hour, here's what I think of them as.
Mistystar is nearing 100, but still looks young because of the magic flower in RiverClan territory.
Graystripe is 89. Brightheart & Co are in their 70-80s.
Bramble, Tawny, Storm, and Feather are
(would be) in their upper 60s. Leaf, Squirrel, and Crow are in their lower 60s, maybe upper 50s.
Jayfeather's generation is in their mid-40s, with Willowshine older and Kestrelflight slightly younger.
Dove and Ivy are upper 30s to lower 40s.
Alderheart is upper 20s, Twig and Violet are 25ish.
Bristle is 19, Shadow is 18, and Root is 15.
Obviously cat years don't translate over to human years all too well. This is based mostly off of how old they feel to me.