Style wise I see Rantanen as a player with high probability of aging well. He doesn’t really rely on speed and although he is big body, he excels more in protecting the puck than actual crash-and-bang physicality, so there are no obvious red lights that would make him very vulnerable to aging.
Of course, since he isn’t especially fast, loss of speed could be devastating by rendering him way too slow to contribute, but getting in to space to score, nice shot and big ass to keep defenders at bay are not lost with age.