I start early with mine but it's pretty much practice for the real thing when they need it. They just get rasped a bit, get used to the farrier, see all the new things he brings with him, etc. The first true trim usually happens once they've been weaned and then they are every 5 weeks with the rest of the gang if it's needed. If not, we let them go a couple more weeks and my farrier will pop in when he's in my area and take care of it.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.