How can youth and wisdom not coexist according to "Through the Tunnel"?
Lessing isn't necessarily saying that youth and wisdom cannot exist together. Rather, what she is trying to suggest is that teenagers reach a stage in their lives where they need some kind of initiation into manhood where they can do something to prove that they have left their childhood behind them and are ready to be considered a man or a woman.
If we think about the concepts of youth and wisdom, we can see that adults are not necessarily presented as wise in the text. Note the following description of Jerry's mother and her constant concern about her parenting skills:
She was thinking, Of course he's old enough to be safe without me. Have I been keeping him too close? He mustn't feel he ought to be with me. I must be careful.
Her insecurity reveals that becoming an adult does not necessarily make you wise straight away. Of course, Jerry does present himself as being a rather young and immature boy, especially when he plays initially with the French boys and plays the fool for them to cover his embarrassment and inability to find the tunnel. However the text isn't about the inability of wisdom and youth to coexist. It is about the need to have some kind of ritual that will allow us to pass metaphorically "through the tunnel" from childhood to manhood.