I think my first round of antibiotics was when I was 13 or 14. I didn't have them again until around 19, and I already had a lot of symptoms for crohn's at that point. I feel very lucky that I didn't take them more often, or my gut would have probably been worse. Antibiotics really mess up your gut flora, so I can see how they could be the gateway to bowel problems.
I have a cousin who took some sort of antibiotics for acne when he was a teen. He's had all sorts of gut problems since then, but no diagnosis other then IBS.
35 years old, Crohn's disease for 15 of them
Resection of ileocecal valve on 09/22/09
Current Meds: LDN, Pentasa, Effexor XR
Supplements: 3 kinds of Fish Oil, Multi-Vitamin, Vitamin D, Calcium, CoQ10, Enzymes
SCD since 12/01/08 - eating Stage 3 foods