Assuming that juvenile hermit thrush has a contrastingly-rufous tail, this isn't a juvenile hermit thrush (nor an adult hermit thrush). This bird doesn't have a fleshy gape that's apparent in this photo, but it does have some odd pale fringes to the back-feathers, so that might make it a juvenile - dunno.
Hi Butty, I think I can answer your question. This fringing is not typical of any plumage/age class of North American Catharus thrush - my suspicion is that it is an artifact of the light catching the feathers just so. Or it's just a weird plumage anomaly. As to other reasons for aging this Veery as adult: all the Catharus in juvenile plumage do have extensive spotting on the upperparts but on the centre of the feathers, not the fringe like this bird. During their first moult (commencing ~ a few weeks after fledging) they would only retain some of the wing coverts and replace all/almost all of the rest of their upperpart feathers with "adult" feathers so you (in theory at least) wouldn't ever end up with a situation where it has the spotted juvenile back feathers but not wing coverts.
As to the actual age if this bird I think it is safe to go a step further than Microtus and say it is an "after-second-year" ie hatched in 2019 or earlier. This is due to the excellent quality and broad tipped shape of the flight feathers and lack of any sign of a moult limit in the wing coverts that would indicate hatching last year.