I believe people who have studied this have found that scorers tend to peak in their early 20s. Around age 23 -25.
As for Gretzky and Lemieux, here's their 100 ES-point seasons:
| | Year | Age | ES Points |
1 | Gretzky | 1982 | 21 | 147 |
2 | Gretzky | 1985 | 24 | 146 |
3 | Gretzky | 1986 | 25 | 143 |
4 | Gretzky | 1984 | 23 | 135 |
5 | Gretzky | 1983 | 22 | 132 |
6 | Gretzky | 1987 | 26 | 124 |
7 | Gretzky | 1981 | 20 | 104 |
8 | Gretzky | 1991 | 30 | 103 |
9 | Lemieux | 1989 | 23 | 102 |
10 | Gretzky | 1989 | 28 | 100 |
10 | Gretzky | 1981 | 20 | 100 |
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So you can argue prime vs prime (assuming a 12-year prime for Gretzky), but more accurately it's the apex/peak/best of Mario's prime matching up against a declining/off-peak/end of his prime Gretzky who had tailed off.
Lemieux may have edged his way into fringe Gretzky prime territory, but he never approached peak Gretzky.