I have a brompton which is about 7 years old. Do I need to think about replacing the aluminium clips that lock the frame in place when unfolded?
The Brompton owner's manual states:
As the manual itself notes, that is an extremely conservative number, but considering the hinge plate itself is a £4.00 part it seems better to err on the side of caution.
Aluminum in general has a useful fatigue life of about 5 years. Which is why most manufacturers have started limiting frame warranties to 5 years. Fatigue Life data for 6061 alloy here.
If your folder has been ridden regularly over the 7 years you've owned it, then consider replacing them. If it's been lightly used, or mostly garaged, then you're are probably ok.
There is no way to check the soundness of the metal short of an ultrasound or possibly X-ray testing. So if you are concerned, which I gather from the fact that you asked the question that you are, then just change them. it will be worth the piece of mind.