Is a mange tout merely an immature pea and there's nothing more to it than that? And while we're at it, is a courgette (zucchino for Americans & Italians) just an immature marrow and that's all?
Does variety come into it?
If mange tout1 is the same as what we call a "snap pea" or a "snow pea", it's not that it's immature, it's a different variety. (Well, I generally eat them before the peas fatten up, so they are immature, but I don't do this with "garden peas" (aka "English peas") because the pods aren't very tasty.)
Hmm... what's a marrow? It looks like a member of C. pepo, but an image search turns up all kinds of wildly different squashes.
1 That's French for "eat [it] all" -- I guess my French is better than my [British] English?