It's very hard to say without seeing him/her in person. Most of my hatchlings reach adult size in 6-7 months(i.e. 60-70 grams and 8"-9" long). If it is 4-5" I would guess it is somewhere around 3 months or so. Most leo morphs (and normals) go through a change in pattern as they mature and the stage at which they are at in that transformation can be a clue to age.

John Yezbak
North Coast Geckos