Early boarding as an ‘extra charge’ is purely a revenue-earning gimmick that is almost un-workable in the low-cost carrier market with its reduced turn-round times.

In the case of a genuine need for assisted boarding, then this should be declared at the time of ticket purchase and re-iterated at check-in (where this actually exists!). Otherwise, you need to be in the Usain Bolt class of athlete to win your desired seat.