Look at most elite performers and they are wayyyy faster at 26 to 30 than they were at 18. Most of them are faster in their early 30's (if, injury permitting, they are still running) than they were 18. This is because of consistency. They have trained and competed their whole life, essentially.
At 18 your average runner is faster than they are at 30 because they train, they are young and recover quickly, they pick up conditioning faster. By 30 your average runner has not been taking it nearly as seriously, and the stress and time constraints of life take their toll on fitness.
A 30 year old who has CONSISTENTLY trained since they were a teenager absolutely SHOULD be faster than they were at 18 because of the extra 12 years of training. You aren't in your prime at 18 - most people hit their natural prime between 24 and (if you are lucky) 30, so let's say on average 24 to 28. By 30 you haven't sufficiently come down from that "natural" fitness to regress to a level lower than that when you were a teenager.
30 vs 18: more strength and power and endurance at 30, a little more natural quickness/speed at 18. 30 wins because an extra 12 years of training. HOWEVER if you haven't kept up speed training, or even proper training or competing fullstop, 18 will win because you cannot make up for a wasted decade in between.
SO most people had the capability to run faster at 18 than they do at 30 because at 18 you have natural speed to draw on despite a lack of conditioning and being a few years away from your prime. BUT if you are and HAVE BEEN a runner (see runners at national to international level, almost all of whom are faster at 30ish than at 18ish) then at 30 you have a LIFETIME of conditioning to your advantage, plus all the conditioning and training you performed whilst AT YOUR PRIME IN YOUR MID-20s which will still be with you to an extent.
If you have wasted your 20's then, most likely, no you will not be able to smash your high school PR's. If you kept it up a bit, you have a chance with HARD WORK, INCLUDING SPEED WORK.