Honestly, Heero's used to people being weird about his age. It had been an asset in the war. People constantly underestimating him, because they look at him and see a child. Most of the people who'd made that mistake are dead now.

A few are now allies of a sort. War works in strange ways.

"I'll cover you." It'll give Heero a chance to see how his new teammates work in action. Even if only a little. It's better than nothing and will give him a better grasp of how this might go in the future.