I noticed the Wiki has them listed as 15, as of 2000. I've never seen that movie in Japanese though, so I can't verify that myself.
They don't seem like teenagers though.
They look older than Brock, who is fifteen, and act like immature adults. Their backstories don't seem to match them being fifteen either.
In one episode of the dub Jessie said she was 17, but most people think she was lying.