Most Olympians are over age 18 said: Many lifelong runners don't even reach their peak until age 25+.
the age at which athletes reach their peak increases as race distance increases. there are many different ways to measure this:
1. the average age at which athletes earn their first medal at a major championship increases as race distance increases. for example, at 400m the average age at which athletes earn their first medal is 23 years (mean 23.44, n 177, SD 1.84), whilst at 10,000m it is 25 years (mean 25.83, n 145, SD 3.02).
2. the age of both the youngest and oldest athlete to medal at each distance increases as race distance increases. the youngest 400m medalist is Terry Tobacco (CAN) at 18y 158d, whilst the youngest 10,000m medalist is Bruce Kidd (CAN) at 19y 121d. the oldest 400m medalist is Mark Kiprotich Mutai (KEN) at 32y 200d, whilst the oldest 10,000m medalist is Dieter Baumann (GER) 37y 179d.
3. the youngest and oldest age at which athletes set world records in the middle-distance events increases as race distance increases. for example, in the 400m, the youngest man to have set a world record is Grover Klemmer (USA) 20y 105d whilst the oldest is Michael Johnson (USA) 31y 347d, while in the 10,000m the youngest man to have set a world record is Richard Chelimo (KEN) 21y 75d and the oldest is Viljo Heino (FIN) 35y 184d. it is also worth noting that Johnson is the only athlete over 30-years-of-age to set a world record at 400m, but five different men over 30 have, between them, set nine world records at 10,000m (Ilmari Salminen (FIN) 34y 200d, Viljo Heino (FIN) twice 35y 184d, Emile Zátopek (TCH) twice 31y 255d, Pyotr Bolotnikov (URS) twice 32y 156d, Fernando Mamede (POR) 32y 243d, Yobes Ondieki (KEN) 32y 139d).
what this says is that your point is correct for a variety of reasons, almost no one reaches their peak at age 18 and most do it in the mid to late 20s. some do it in their 30s.
cheers.