To clear things up, the ages I'll list here are the ages I usually write the characters in.

BUT if you want my opinion on the show canon, I'd say that they were at that age during seasons 5 or 6, maybe 7.

Lloyd- 8 (16 after Child's Play)

Kai- 19

Jay- 18.5 or 19

Cole- 21

Zane- Technically like 43 in nindroid years but let's go with 22? Idk I'll leave that up to you...

Nya- 17

Pixal- Less than 5 in nindroid years, let's go with a good ol' 20 or 21