I doubt Pouille hit his ceiling. No player maximizes their potential at 23y these days. He just fares through a rough period.
Maybe, maybe not. First of all, it depends upon what you mean by "maximize." Historically speaking, most players tend to reach their absolute peak in the age 24-25 range, but reach their prime in the 20-22 range.
But because you said "these days," I'm thinking you mean prime and not absolute peak, because players not reaching their absolute peak until 23-25ish isn't anything new. There are outliers, but 24-25 is the historical peak for most players. We've talked about this before, and while I tend to agree
somewhat with "later peak theory," I don't think the evidence is conclusive. Yet. Why? Because it is only based upon the weakest cohort of players in Open Era history, the guys born 1989-95ish. Most of the top players before--from Rafa, Novak and Andy, but also Del Potro and Berdych--reached a high level (prime) in the age 20-22 range. The younger generation--players born 1995 and later--are just entering that 20-22 prime window, and the only player who looks like a surefire Slam-winner--Alex Zverev--is already showing what could be close to prime form at age 20.
So in other words, the notion that "no player maximizes their potential [reaches prime form] at 23 these days" is only supported by players born 1989 to maybe 1994, that is players turning 23 to 28 this year and who are members of the weakest cohort since the weak group between Rod Laver (b. 1938) and Arthur Ashe (b. 1943) and John Newcombe (b. 1944).
To put that another way, we're going to at least need to see the guys born in the mid-to-late 90s turn 22 to assess whether the beginning of prime range has really been pushed back significantly, meaning we'll have a better idea by 2019 or 2020. I personally like to use first big title as signifying a player entering prime form (prime = close to and including peak, before or after), which means Zverev would now be in his early prime - although probably no more prime than Roger Federer was in 2002, so a "tender prime" (I suspect Sascha will get deadlier over the next few years). But we're going to need to see when his better peers win their first big title, namely guys like Kyrgios (already 22), Rublev and Fritz (both turn 20 in Oct), Tiafoe (19), Shapovalov (18), and also possibly Khachanov (21), Medvedev (21), Coric (21 in Oct), Ruud (18), Donaldson (21), etc.
It could be that the "weak generation" extends through 1996, and Zverev marks the beginning of a stronger group. If that is the case, we won't know if "late peak theory" is true until 2019 at the earliest, when 1997 players turn 22 and should have reached their prime years.