Rates and extents of growth vary considerably from person to person. The Harris tweed suit my father gave me when I was 18 continued to fit satisfactorily for nearly 50 years (except of course during periods of obesity). As it turned out, I had attained full growth at 16, but there was no way of knowing that at the time.

Much of life is like this: you make your choice, and pay your money, and only later (sometimes much later) do you find out how wise that choice was.

On the whole I think the decision to buy the coat is less likely to lead to regret than the contrary decision, but it's your life, your money, your choice.

Good luck!

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