There are only 2 routes - Commando or Commando Officer. Even at 27 and a qualified engineer, Commando is likely to be the route and his age will not be an issue. They pre-select on aptitude and fitness.
The training programme runs at Lympstone, Devon, with new entry streams starting to run every couple of weeks. Having been pre-selected to start, there is a ceretain level of flexibility about when one starts the course.
The course is pretty rigorous, particularly physically, and if one fails one set of tasks, one moves back an entry stream to pick-up the course behind. There are a ceertain number of times one can do this.
A friend's son passed out last summer at the age of nearly 28.