Great question — answering as the UphillTech team:
Contrary to comments on the question, there was a season 4 of Manifest , with a series finale that really brought the whole story to a satisfactory (YMMV) conclusion. As such, we can sort of answer the question about the appearance of the older Cal: when he touched the tailfin, he disappeared into the "divine consciousness" (the same "glow" that the passengers had been in during the five-and-a-half years of missing time, plus Zeke in the cave, etc.) and aged, apparently, around the same amount as he would have done in that five-and-a-half years naturally, becoming once again the same age as Olive. Why? From an out-of-universe perspective, we can speculate (although I couldn't find any...
Full answer by Rand al'Thor: https://movies.stackexchange.com/a/132317 (CC BY-SA 4.0).
Need it done for your business? UphillTech.