It's certainly not that simple. Yes, there are hormones in
(some) meat and dairy products, and hormones are involved in
puberty. However, this isn't enough information to conclude that
hormones from food reduce the age of puberty.

In many countries in Europe, there are NO hormones in meat and
dairy products. From what I can see, the onset of puberty is
occurring earlier and earlier here as well. 10 or 11 for girls is
not uncommon.

The general reason is probably better health and nutrition in general,
or at least MORE nutrition. As Brad mentioned, low food intake leads
to an extended life span AND youth span. So, it is not surprising
that high intake shortens this span.

It also makes sense from the point of view of evolutionary biology.
If food is good, grow up and get pregnant as quickly as possible and
take advantage of the good times while they last. Other species
take this them to more extreme implementations, such as producing
almost all male or almost all female offspring depending on climate,
food supply etc.

To get back to Brad's first point and the replies to it, there are in
fact birth-control pills which one can take without a pause. (The
usual plan is three weeks of pills followed by a week-long pause,
during which menstruation occurs (some pills might have placebos for
this week, so that one doesn't forget to start up again after the week
but just continues to take them.) In other words, they suppress
not only ovulation but also menstruation permanently.

Of course, this is not good in the long term. Birth-control pills are
not good in the long term and other methods are to be preferred if
possible. (This is not surprising; the female body is made to be
pregnant or nursing and this can't be simply switched off; there is a
lot of evidence that lack of ovulation, pregnancy and nursing is one
of the causes of breast cancer.) However, it is relatively safe
and easy to just leave out the monthly pill-break (menstruation)
when going on a summer holiday or whatever.