Dylan Kelbold was 18. That's an adult according to the legal system here in the US. Eric Harris was 17; several *dozen* felonies later (most with 'extra circumstances' including significant weapons charges [sawed-off shotguns are a big no-no, pipe bombs likewise] and wearing masks!), you can be sure he would have been tried as an adult. Note that both of them had been through the juvenile detention system for property crimes; no doubt they had other (non-prosecuted) criminal histories.
/jordan