The average 20-something would be way above 13 sec.
I think you're way optimistic on how fast "most people" could run.
I'd guess with a year of training, maybe 2 out of 3 twenty-something adults could break 17 seconds. Keep in mind that about a third of the population is obese and couldn't run 20 yards at the start of training and probably an equal amount will get injured in the training process.
Based on a 2105 sample of 15,507 HS seniors in the US, the mean 100m performance was 12.1 sec. 15% were faster than 11.3. These are self selected sprinters...not the average 17-18 male.
The OP's 12.38 is way above average for males even with a year of training.