Coming over at age 20 and 24 is not the exact same thing.
True. At age 20, Peter Šťastný was still a little short of being considered a top 10 player in Czechoslovakia. At age 23/24, he was voted Player of the Year. Everything else being equal, a player coming over at 20 will usually take a bit longer to develop.
Still doesn't explain why Stastny was able to perform at an elite level asap.
He had just entered his prime and was still young enough to fully adapt. One could probably say he was in the perfect age to come over. As pointed out by Canadiens1958, he also had one of his regular linemates from Czechoslovakia with him the moment he stepped on NHL ice and from the second season on he had both his familiar linemates of the past few years with him. It's hard to imagine more favourable circumstances for him.
You want to use Mogilny as an example to prove your point? He didn't have anywhere near the nhl career that Stastny did.
How their careers panned out is one thing, what calibre of players the proved to be in the NHL is another. Mogilny didn't have Šťastný's consistency, just like Bure didn't have Šťastný's health and longevity, but at his best Mogilny was still was a top 10 scorer and one of the most successful goal getters in the league. This young generation of Soviet players had the time to adapt and did so successfully, regardless of whether their overall careers went as smoothly as that of Peter Šťastný or that of Jaromír Jágr and Teemu Selänne.
Let's see. Which forwards who came to the NHL from the Soviet Union and Czechoslovakia between the mid-1970s and early 1990s had the most impact? Probably these players:
Jaromír Jágr –
18-19 in his first NHL season. At his best #1 scorer in the league. Multiple Art Ross trophies.
Aleksandr Mogilny –
20-21 in his first NHL season. At his best #7 scorer in the league. Best goal scorer in 1993.
Pavel Bure –
20 in his first NHL season. At his best #2 scorer in the league. Best goal scorer multiple times.
Sergey Fyodorov –
20-21 in his first NHL season. At his best #2 in league scoring while being Selke-calibre defensively. Hart winner.
Žigmund Pálffy –
21-22 in his first season in NA. At his best #9 in league scoring.
Peter Šťastný –
24 in his first NHL season. At his best #2 in league scoring. One of the best NHL scorers of the 1980s.
For comparison, here are the most prominent players from the same two countries whose NHL impact was limited or at least below exceptations:
Igor Larionov –
28-29 in his first NHL season. At his best #42 in league scoring as a good two-way center.
Václav Nedomanský –
30 in his first season in NA. Never top 10 in WHA scoring.
Milan Nový –
31 in his first (and last) NHL season. #130 in league scoring.
Sergey Makarov –
31 in his first NHL season. At his best #27 in league scoring.
Ivan Hlinka –
31-32 in his first NHL season. At his best #81 in league scoring.
Players from the second group impressed fans and teammates with their skill level, but didn't have the scoring impact the younger group had. You don't see a pattern there?
Again, I'm not saying 20 and 24 is the same. While still being young, Šťastný already had a few years of experience in international hockey under his belt. That's certainly one of the main reasons he was able to hit the ground running in the NHL while players who were even younger like Bure or Jágr took a little more time to improve over the course of their first two (or so) seasons.
Peter Šťastný before turning 24:
Ranked among Czechoslovak players: 1st, 6th, 12th, 12th, 15th.
Ranked among European players: 8th.
World Championship awards: none.
Sergey Makarov before turning 24:
Ranked among Soviet players: 1st, 2nd, 4th, 4th, 9th.
Ranked among European players: 1st, 4th, 4t, 6th.
World Championship awards: Best Forward 1979, All-star team 1979 and 1981 and 1982.
Maybe it means Stasny was being overlooked in his European Career.
You'd not only have to argue Šťastný was being overlooked internationally in Europe (I'd sure like to hear your argument why that was, BTW). You'd also have to argue he was being overlooked in his own home country, by the very same people who put him on their national team and who voted him Player of the Year in 1980. Šťastný was ranked
outside of the top 10 in Czechoslovakia in four years out of five. Makarov was ranked
among the top 10 in the Soviet Union with its deeper talent pool five years out of five and
among the top 5 four years out of five.