LOL - the trainer is really pegging age to neuter against leg lifting?
Your vet is right, it is largely a learned behaviour. My guy started at about 4 months (having observed other males), some dogs don't start until 2 years or so. And some never lift their legs. So it's a heck of a lottery if you use that as a guide (and besides which, leg lifting isn't actually a desirable behaviour
- especially if it turns into marking, which
is testosterone-linked).
About 6 months seems to be the most common age for neutering. That's partly convenience though, and anywhere up until about a year old seems to be the most common veterinary recommendation. A short article on that here:
http://www.peteducation.com/article....&articleid=911