I think this question is half about the history and half about the science of puberty.

Trends toward earlier puberty are well documented starting in the 19th century [1] and especially from the mid-20th century to date [2]. Wherever nutrition and socio-economic conditions have been sub-optimal and then subject to improvement, a decrease in the age of puberty has been seen[3]. Due to the complexity of the question, it is almost impossible to be 100% sure that nutrition and only nutrition is responsible for this change. The impossibility lies in the fact that no one will agree to a study where you intentionally malnourish children to see if that delays puberty.

I did a cursory search of the literature trying to find older sources for the age of puberty. I found some tidbits that you might find interesting:

In Leipzig around 1727-49, the age of puberty on males was probably about 18.0 years. This is judging from the change of voice (cracking voice) noted in Bach's choir boys[4].

According to Aristotle the first seed (menstruation) comes toward the end of the fourteenth year but remains sterile until about the seventeenth or even twenty-first [5]

1. Wyshak G, Frisch RE. Evidence for a secular trend in the age of menarche.N Engl J Med 1982;306:1033–5.
2. Euling SY, Herman-Giddens ME, Lee PA, et al. Examination of US puberty-timing data from 1940 to 1994 for secular trends: panel findings. Pediatrics 2008;121(Suppl 3): S172–91
3. Ong KK, Ahmed ML, Dunger DB. Lessons from large population studies on timing and tempo of puberty (secular trends and relation to body size): The European trend. Mol Cell Endocrinol 2006;254–255:8–12
4. Daw, S. F. "AGE OF BOYS'PUBERTY IN LEIPZIG, 1727–49, AS INDICATED BY VOICE BREAKING IN JS BACH'S CHOIR MEMBERS." Human biology (1970): 87-89.
5. Eyben, Emiel. "Antiquity's view of puberty." Latomus 31.Fasc. 3 (1972): 677-697.