I've seen a lot of people clone early with no problem..
Here's the quote about cloning from Jorge Cervantes "Marijuana Horticulture Grower's Bible"
"Mother Plants: Any plant can be cloned, reguardless of age or growth stage. Take clones from mother plants that are at least two months old. Plants cloned before they are two months old may develop unevenly and grow slowly." It goes further to talk about cloning from a flowering plant..