So, you are saying when Washington first (from our perspective) meets Neil (Pattinson) at the club, Neil has previously "finished" inverting himself and is now back into the "normal" flow of time. Right?
Yes. And no, you don't un-age traveling inverted. You still experience aging and the progression of time, you're just inverted compared to the rest of the world. As far as we know you can only travel forward (normal time) and backwards at the same pace (it takes an hour of your time to travel backward one hour).
Which is why, like I said before, I don't think Neil can be Kat's son. He would have had to spend 15+ years inverted after he grew up, got his PhD, trained, and had adventures with the an older version of the protagonist.
- "Great spirits have always encountered violent opposition from mediocre minds." - Albert Einstein