How many 18 to 20 year olds were killed in the Afghanistan War?

None from the United Kingdom nor the Unites States, and I suspect neither the rest of NATO. Some Afghan people may begin fighting at young ages.

However, both the UK and the USA allow certain under-18s to enlist. Both countries have policies forbidding the deployment of under-18s to any combat zone. However, it has come out that several dozen under-18s were deployed to Afghanistan during 2003-2005; in both countries' cases, the deployments seems to be anomalies (either mistaken orders, or mass deployments of units without proper checks on eligibility), and, so far, have not been repeated.