you know, I haven't read or seen many Remus/Tonks fics but if I picture it, it doesn't seem that strange to me, especially comparing it to some of the other pairing. XD Tonks could be older than 21 (although she seems like she acts like a teenager sometimes) and i'm sure that with her metamo-whatever abilities (ah! Is Jan actually skiving up on looking up this magical term? The sky must be falling!) she can look as old or as young as she wants. Imagine her appearing as old as Remus' mother! XD