In some ways it can be advantageous to join under 18. Besides the Commando Logistics posting/draft, you are essentially available to undergo additional courses whilst waiting to become operationally deployable.
A lot of guys have gained their HGV licence after RT, some get sent to the Commando Display Teams who tour the country, giving first class demonstrations of rapid-roping from helos, unarmed combat, section attacks, fast boats and loads more. (RM Recruiting Officer is the boss on the CDT and is doubtless better placed than I to tell you about it.) Other guys undergo specialist or trade qualifications earlier than others, some join the RM Visibility Team touring a UK region, visiting schools & giving team building/demos/presentations/climbing/activity days and suchlike.