He doesn't mention how early, but he does say "immediately as they become young." Whatever that means.
1. If parents delay marriage and it "leads to sin, the sin would be on them."
2. To garner respect for the in laws.
Parents should conduct the marriage and bring their daughter-in-law, as was our way in the past. That is why the daughters in law used to respect them and thought of them as her parents. When the son himself bears all the burdens of marriage, how can his wife feel obliged to respect his parents as her own. She would rather be unhappy with their presence. "They are disturbing our privacy." That family system will be disrupted completely. Marry your sons early, and bring his wife as your own daughter.