No doubt Gretzky was getting a bit washed-up by 1998-99, his last season (although he still had 55 points in 54 games and was leading the Rangers handily).
But look what he did in the last 40 games of the previous season, from Jan. 3rd 1998 to April 18th 1998:
1. Jagr - 55 points
2. Gretzky - 54 points
3. Palffy - 50 points
4. Allison - 43 points
5. Turgeon - 43 points
Forsberg, btw, had only 40 points in this period, but in only 34 games. That's projects to 47 in 40, well behind Gretzky's pace. (Gretzky at this point is 37 years old, and on a .415 team with one of the worst offenses in the League.)
That's the thing with Wayne and Mario after they were 30 or 31. We can find stretches of 20 or maybe even 40 games where they got back to almost their prior prime form, but it was hard for them -- like any forward (except maybe Howe) -- to score as consistently as before that.
Gretzky was still lights-out in the 1993 playoffs, much of 1993-94 -- especially the first half of the season (he was on a 150-point pace at mid-season), the first 19 games of 1995-96, the first 36 games of 1996-97 (leading the League in scoring at late as Dec. 17th), the 1997 playoffs, and, considering his age, the back-half of 1997-98, although by then he couldn't score like of old.
Mario was unstoppable up to his 31st birthday (which was the start of the 1996-97 season). But he dropped about 40-50 points off his prior season's pace that year (partly due to the Pens scoring 35 fewer PP goals). Still, from Dec.17th to Jan.26th, he suddenly put up 42 points in 17 games. Three-and-a-half years later, when he came out of retirement, he famously scored 76 points in 43 games... but then already looked gassed in the playoffs. Similarly, two seasons later (2002-03), he put up 44 points in the first 21 games, but then really fell off towards the end (18 points in the final 23 games).
We should bear in mind, then, how incredible it has traditionally been for any high-scoring player to maintain his consistent, elite touch past age 30 or 31, when even the two most talented players ever couldn't really do so.