So in PoR casting haste on the party ages everyone by 1 year, and I read that it will start affecting your characters if they get too old. Is there a way to undo magical aging? Is this also in later games? I plan on taking my party to the other games of the series so should I just use haste very sparingly?

There are Potions of Youth in some of the games. But I'm not so sure the aging thing was actually coded.