Levels are aleady taken into account when calculating the amount of bots that will spawn. But it's obviously not enough. Someone under level 20 has barely any use for a DTV team, and should add 0 bots.
The major reason I don't play DTV more is that you need an organised team with only good players in order to beat the waves 5+. Generally, the proportion of good players is something like 20%, because DTV actually attracts all the leechers.
So I agree with the suggestion, but I would prefer a version with level 20- people allowed in the server without adding any bots.