As in all, that varies
As some guys stop growing at 14, and some at 20 (height etc) I suppose the penis is no different.
If you are 18, it’s possible it could grow more but personally I doubt it. On the other hand, I would be VERY wary of any PE exercises I did. Should you consider PE at this ‘early’ age, please consider everything very seriously, beginning with the TRUE reason why you want to do PE.
Along a different path, it seems the young guys don’t gain as much as the older ones, so I would be very patient and remember that you could always start in 2-3 years time and still have plenty of time to grow a monstar dick for the best sexual years of your life!