Lucas will age faster. And he has all his memories from his past lives. He'll start talking (clearly and like an adult) at around a year and will look adult (by our standards) at around 12-14. He ages between 2 and 3 times as fast as a normal human but in some things he'll be even more advance simply because he doesn't have to learn things like walking and talking, just wait until his brain/muscles/etc are developed enough to do it. Once he's mature he'll age like a normal human.
(had to figure that all out to figure when Joe and Lucas could have their first conversation which will be the next phoenix scene to be posted)