I see from a previous post you are Army and wished to transfer to RM ? This would therefore be your first avenue.
Royal Navy Aircrewman (ACMN) are selected form within the RN and RM, namely you come from a source “branch†(Cook Mechanic Seaman etc) an upper age limit does apply 27 I think.
There are two strands to ACMN branch. Jungley (Commando Helicopter force, Sea King MK4) this is about 50/50 Marine / Navy. The other is Maritime Aircrewman which means Merlin in ASW / ASuW roles.
I here there is a RM on Merlin course at present.
In sum suggest you approach your system to start a Transfer to RM (or in deed RN) After that you will have to compete for an Aircrewman place, I imagine you will need to complete at least one operational tour as a Booty before being considered for ACMN. That is unless you already have a transferable skill such as Air Gunner.
Regards