I think it's important to draw a distinction between a player's "peak" (which is often over-emphasised) and his "plateau" (which is to my mind far more important).
It is rare for players, who go on to have a good Test career, to have more than a few games early on where they're not Test standard players. That's very distinct from yet to produce their very best performances. For most players, their very best will last for only perhaps a year or so, or else will only be produced in very small (2-3 game) spurts.
To give the example of Michael Atherton, because his case fits well with near enough everything I've talked about:
1989 (2 games) was his "novice" spell - he was picked too early and wasn't up to requirements.
1990-1992/93 was his "inductory" spell - he was mostly a Test-standard batsman in this time, but he was beginning to show signs of his fallibility to his back condition and did not produce his best.
1993-1996 was his "peak" period - a way to suppress the problems had been ascertained, and he was barely out of the runs.
1996/97-2000/01 was a past-peak-but-still-on-plateau period - there were times when the injury could no longer be suppressed and when that happened (Zimbabwe 1996/97, Australia 1998/99) he was a walking wicket, but otherwise he was mostly still good for runs apart from a couple of consecutive series' in 1997 and 1998 where he struggled.
2000/01-2001 (10 games) was his "decline" period - he was no longer good enough any more and struggled against everyone he faced.
But more broadly, 1990-2000 (in terms of calendar-years) was his "plateau" period - throughout, he could be said to be (when not operating under grossly reduced fitness) a Test-class batsman and a pretty decent one at that. He played 90-odd games when fit in this time, and it is that, not 1989 or 2001 or the brief periods where he lost fitness, on which I judge him.
Most players will feature some of the above, though not that many will feature everything Atherton did. It is incredibly rare for a player to have a lengthy Test career of being Test-class throughout and not endure any significant fluctuations in productivity.