Same as general service. You must commence initial training before age 37 for non technical ratings, before age 30 for technicians. Officers likewise reflect the same age parameters as their general service counterparts.
Yes, the upper age for technician was reduced from 37 to 30 a couple of years back due to the fact there was a requirement to ensure adequate numbers joined early enough to reach CPO rate and complete a couple of sea drafts.
ET(MESM) is, off the top of my head, about a twelve month wait, ET(WESM) is about half that. Don't take my word for it though -double check with your AFCO as there are 50-odd jobs and the waiting times fluctuate frequently.