People in their 40's still fare quite well even in their 40's, so I don't see any potential problem with that as long as they are in decent physical condition. The one thing to be concerned about (at any age) is your knee and ankles since TaeKwonDo is primarily focussed on kicking techniques.
I know quite a few people (my dad included) who had tested for their black belts in their 40's, so it is surely not an issue even for a beginner to pick it up.
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