Unintentional injury (likely mostly car crashes?) seems to be the leading cause for all age group up to age 44. Then come suicide/homicide which only start declining in the ranks in your 40s, probably because other causes (cancer, heart conditions) start to take over.

I'm in my 40s and officially more likely to have my heart kill me, than myself.

https://wisqars.cdc.gov/fatal-leading