Can’t give you an approximate age (as I don’t measure their ages, just go by weight and sexual maturity), but I can say that “she” may not be a she in a few grams. Your gecko is right on the cusp of sexual maturity. Generally around 15-18 grams is about when testicles begin to drop – although testicles could still drop at 20g. Once your “girl” hits 20g and still has no signs of a bulge, she’ll be mature and have her “adult colors.”
Honestly, there is a lot of talk of SVL (snout vent length) in the age of Cresteds…but that isn’t a really good measure of “age.” For example, I have a Crested acquired back in April of last year which finally made it to 9g. It sat between 3-5g for about 7 months. Going by just length (or even weight), it could be have been mistakenly classified. Granted, that’s not always the case – but it’s a good example to not count on length. 
Feeding schedule is 3 times a week with CGD (mixed with water only – otherwise it throws off the nutritional balance). Or 2 times a week CGD and 1 time dusted insects; even 2 times dusted insects and 1 time CGD is ok. Feel free to “mix it up” a little and see what your geckos respond to best.
To be honest, I don’t know what a 30L tank is, we use Sterilite tubs for our groups and breeding projects. Height is very important to Cresteds, as they are arboreal – so think of maximizing vertical space more than horizontal.
Ventilation is good, and so is humidity. Try to mist down the cage once (in the winter we have to do it up to three times) a day; if problems with shedding occur, up the moisture a nudge.
Cresteds are very tolerant of handling. However, they are quite the “pouncy” little buggers and rarely hold still for very long. I’m sure your Crested doesn’t “seem” like it wants to go back in the cage at night – but little more goes on in a Crested’s head besides “pounce, eat, drink, poop, sleep” in that order. 
Last thing: Pick up “Rhacodactylus: The Complete Guide to their Selection and Care” by Philippe de Vosjoli, Frank Fast, and Allen Repashy. Many answers you have as a Crested owner can be found there.
Good luck! 
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