Q. how many underage alcohol related deaths were there since 2000?

For factual reference questions like this I generally start with Google. Check the top 10 Google results for this search:

underage alcohol death statistics site:.gov

When I do this search teh results list several CDC fact sheets and other government websites with more information on underage drinking and death.

(Remember, don't get distracted by the (faintly labeled) advertisments for non-government websites)

Please let us know if you need more info...