T.W.
Hi M.,
I love to both bike and run but didn't want to have two separate items to store (bike trailer/jogger) so, I bought a Chariot stroller and it's wonderful. I bought it when my son was 18 months old and I was pregnant with my second. At about 3 months after giving birth I began running with both my kids in it. I ended up purchasing the infant insert for it, which worked out fine because my son was on the bigger side. Chariot also makes an infant hammock that I believe you can put a baby in even younger. The only downfall is that the inserts are pricey and you don't use them for very long, by 6-7 months my son was just riding in the regular seat...so look on craig's list for a used one.
For biking, I waited until my son was 1 year and could wear a helmet, that's what is recommended. My stroller still got plenty of use with running.