Actually in the UK it can be very useful as you need a CMAS 3* if you're doing academic courses that involve diving such as marine biology or archaeology.
If conservation work is being overseen by HSE I would imagine the same would apply.
This is precisely why I am just finishing my CMAS 3* (in Cyprus) - at the grand old age of 50 - it has taken me just over a year and 100 plus dives. In the UK HSE class marine biology as scientific diving, and you require CMAS 3* (or equivalent) to do it for your employment, if you just want to volunteer on projects being run by other qualified persons then you don't, you just need to comply with whatever is required for their HSE assessment for the activities, which I think is pretty much likely to be recreational for the conditions.
I work as a consultant in the environmental field, mainly dealing nature conservation, habitats and marine protection which is my spur to get out and see what I am writing policy about.
But age is no bar, so long as you can still lift and rescue diviers into boats, meet the physical standards and so on then go for it. I guess it was (is) my late mid life crisis.
Good luck - P