There’s not a set age across the board. Some quit growing earlier than others.
The general idea is that puberty has a blanket ending at 18, but that’s definitely not true for everyone. Penis growth doesn’t even necessarily have to be tied strictly to puberty. Puberty is just the bodily phenomenon where most maturation and growth hormones are released, such hormones can still be released in much less concentrated amounts years after a person’s puberty years.
Many men as they get older and into their 30s and 40s find that their penis has gotten thicker or a little longer than it was when the guy was in his early 20s, same goes for noticing their testicles have gotten larger. These occurrences are actually quite a common experiences for men as they grow older, showing that growth hormones are active for long periods of time, and continue beyond just the main boom during puberty years.
I know that compared to age 20, all 3 genital measurements have increased for me. My dick is longer and thicker than it was at 20, enough for me to notice, and my balls are also larger than when I was 20. So to answer the OPs question, yes, you can still get bigger after 18, and I’d say there’s a high chance you will. But what growth you get may be barely noticeable or it could be very noticeable, it’s all up to your genetics and glands.