Andy Robertson was behind Alberto Moreno in the pecking order when he joined Liverpool and appeared in 2 league games before the new year in his first season. Not quite class from the off, and he really hit his best vein of form in the run-in to 2018/19, i.e. when he was approaching his 25th birthday.
We've already covered Walker, who was 25 when he had his first great season for Spurs and 27 when he joined Man City. Very much in the same bracket as Cancelo. So that's 3 of the 4 best full-backs in the league not meeting the bolded criteria in my book, with the obvious exception of Alexander-Arnold who is a generational talent.
Few other immediate examples:
- Shaw hit the best vein of form of his career from late 2020 to summer 2021, after turning 25.
- Trippier joined Spurs at 25 having spent 3 of the previous 4 seasons in the Championship, and joined Atletico Madrid at 29 (winning the La Liga title at 30)
And the important point, which I made in my last post too, is that it's more important to look at actual playing time than just age to assess how far along a young(ish) player is:
- Dalot only played 6 league games for Porto before we signed him, made 18 appearances for us in the first season and played virtually no football in his second. At the age of 23, he's played a total of 4,893 minutes in the English, Portuguese and Italian leagues (the equivalent of 54 games)
- Reece James, at roughly the same age, has played the equivalent of 113 games of league football across the PL and 1 season in the Championship
- Tierney (two years older than Dalot) has played the equivalent of 152 games (Scotland and England)
- Alexander-Arnold (a year older) has played the equivalent of 153 games (all Premier League)
- Chilwell (at the time of his move to Chelsea, which you mentioned was at the same age Dalot is now) had played the equivalent of 114 league games for Leicester. Again, over twice as much playing time as Dalot
- Robertson (since you mentioned him moving to Liverpool at 23) had played the equivalent of 146 league games for Hull and Dundee
All of which is a long-winded way to say there's really nothing to suggest Dalot won't or can't continue to improve, or that he's already close to his peak just because he's 23 and the average is 25.