Can someone explain Tommy and Billy's ages/reincarnation to me?

This is something I don't understand even after looking at other posts on it. Scarlet Witch fans, what's going on here?

• Wanda is apparently supposed to be in her early to mid 30s.
• She had Tommy and Billy in her early (?) twenties.
• Pietro already had his daughter Luna by then.
• Tommy Shepherd and Billy Kaplan-- the reincarnations of Wanda's sons Thomas and William-- are adults when they meet Wanda. They're much older than Luna despite originally being born after her.

So, why are Billy and Tommy teens instead of little kids?