I'm always AMAZED at how the smaller/young bucks will find something to "reach" with, like a log or a bale of hay. The girls will just back right up to them. My problem has been that if there is an older buck, the older does always go for the smellier, more mature guys!
My buckling went through his first "rut" about two days before he was 8 weeks old and he was mounting the does and coming closer than I liked to getting the job done. Needless to say he got his own pen that day. His sire bred his first doe at about 4 months old or so before I bought them together so about four months after purchase I got a crash course in kidding. These are all Nigerians so that might make a difference.