TBH I thought he was, perhaps, mid-teens, but also I read it when I was eleven, at which age mid-teens feels Very Grown Up, so really who knows how old he is. Probably not older than early twenties in Queen of Attolia, given that everyone keeps commenting how young he is.

Rereading The Queen of Attolia after Thick as Thieves is also giving me a VERY different view of Nahuseresh and Kamet.