I agree. When the 12A certificate was introduced, we were told it was because parents were complaining that they could not take younger children to see action-adventure films (Spiderman was the example, at the time).
I don't have a problem with that. I suspect parents are more than able to take a judgment on the suitability of action films for their children, and potential psychological harm is slight at worst.
Unfortunately we've now got a lot of teen comedies in 12A as well. Many of these contain sex references (and, in the case of St. Trinian's 2007, drug-references) that I don't think are suitable for younger children.
To be fair, this isn't necessarily a new phenomenon. Many Carry-on films were "A" certificate, as were the dire "On the Buses" movies. These arguably had equally unhealthy sex references. The difference, though, they were always about adults, not teenagers.