I think part of peaking means being able to translate what you do in practice to your competitive programs.
Yes, I agree with this definition of 'peaking', and so it does actually mean that 'peaking' can occur anywhere between 12-13 (for those who don't survive puberty) to late 20's, depending on the skater.
Maybe 15-20 years ago, one didn't see 'peaking' in mid to late 20's, but several factors make it possible now:
1. availability of nutritionist advice (based on DNA analysis) to know what exactly you should be eating at what times and in what amounts and in what order
2. availability of trainers who are able to specify what types of training to do in what order to gain or lose what kinds of muscles in what parts of your body
3. advancements in coaching techniques (i.e., use of handycams in everyday practice and other computer-assisted analysis of athletes' movements at a microlevel to spot what is not right) that make late-stage technical corrections less troublesome
In other words, there is more precise information on how to micro-manage your health and physique, there are now more ways to continuously improve as a figure skater, and physically, women do have their natural peak around 23-5 and men in their mid to late 20's (if you look at what age groups are winning in other sports, that is what you can say), and since there's such a strong mental component to this sport, some skaters may have physically peaked a bit earlier but only reach their mental peak a bit later, and so as a whole, they reach their 'peak' in their mid to late 20's for women, and men bordering 30. So it's to be expected that 'older' skaters will continue to win in figure skating.
Of course, this is not to say that younger skaters won't win. I'm sure there will be occasions when a precocious skater will win.