Recently acquired a couple of young pines..a Weeping Mexican Pine & a Monterey. These saplings had never been potted & are purportedly 2-3 years old. My question has to do with "topping"..at what age would it be safe to cut the height of the tree...I have already pruned the topmost candle about 50%
I would like to take about 20-25% of the top growth off...???

I am not sure about the specific species. But in general pines can take a lot of pruning at any age. However, I cannot come up with any good reason to hard prune 2-3 year old trees. Normally we focus on trunk thickening the first 5-10 years. After that we focus on tapering with some strategic hard pruning. Start with tapering now and the thickening will slow down very much.