Lisa looks like an adult but I have no idea how old Amber is.

The tall characters like Lisa and Jean are clearly meant to be adults, while short characters like Klee are clearly meant to be children. The mid-height characters like Amber feel like they're supposed to be coded as teenagers, but personality wise, some of them seem to be coded as adults. Characters like Bennet or Barbara have personalities that are more innocent/child-like, and they come across to me a being in the age range for high schoolers. On the other hand, characters like Amber and Noelle come across (to me at least) as much more mature than a typical high schooler, and both of them have jobs that I'd regard as too difficult/professional for most teenagers (Amber being a knight and Noelle being a hyper-competent maid who's training to be a knight), so I've been assuming they're probably in their early twenties. However, there's also Xingqiu who comes across as being both mature and competent, while still feeling coded as being high school aged.

In terms of in game lore though, some of the info about ages is a bit unclear/contradictory. For example, in Amber's "About Noelle" voice line, there's a discrepancy where the on screen text explicitly says Noelle is younger than Amber, while the actual audio of the voice line has Amber referring to Noelle as her predecessor, implying Noelle is actually the older one. To make it more confusing, in Noelle's "About Amber" voice line, she says she looks up to Amber, which could imply that Noelle is younger than Amber, but the rest of the line is her talking about how kind and accomplished Amber is, meaning she seems to just look up to her in general. Basically this means the game is intentionally or unintentionally unclear on how old its characters are, even relative to one another, so really it's probably just up to your own interpretation as to how old certain characters are.