This guy started playing at 18
How good you get depends on how you approach the instrument: How open minded you are, how patient you are, how much you practice you can do, how much playing you do, what you want to play. It also depends on your aptitude to music, some people pick it up easily but for others it's a struggle. Starting earlier does not necessarily mean someone will be a better player, in fact someone of 14+ can easily pick up in less than a month what takes the average young child many years to learn.
Most importantly you need to make sure you do things right from the beginning, you only cheat yourself by trying to cut corners.
FWIW I've been teaching guitar professionally for nearly 10 years and I started playing when I was 17.
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