well, not really a great point actually, since it's not true.
Gretzky played until he was 37, Mario played until he was 40.
Mario got cut down in his prime, though. From the 6 peak prime years ages 24-29 he played game totals of 59-26-64-60-22-00. Just crushed his peak prime.
And he was still in his prime when he retired for 3yrs due to cancer. The year before he left, at age 31, he scored at a 132pt pace. The year he returned, at age 35, he scored at a 145pt pace. So those were still 3 prime years he missed at ages 32-34.
If we look at Adjusted Points Per Game per Season:
Lemieux (17yrs): 2.23, 2.17, 2.15, 1.91, 1.83, 1.81, 1.75, 1.67, 1.54, 1.52, 1.50, 1.46, 1.44, 1.39, 1.08, 1.00, 0.81
Gretzky (20yrs): 2.20, 2.13, 2.08, 1.99, 1.96, 1.95, 1.94, 1.87, 1.78, 1.73, 1.64, 1.59, 1.48, 1.47, 1.46, 1.26, 1.23, 1.21, 1.16, 1.00
As for the prime years he missed completely.
Age 28: 1.50
Age 29: missed
Age 30: 2.23
Age 31: 1.67
Age 32: missed
Age 33: missed
Age 34: missed
Age 35: 1.91
So you're probably looking at 4yrs years in the 1.7-2.0 range that he missed completely.