Unfortunately there is no real size/age ratio available as all dragons grow at different rates and some just don't get as big as others for several reasons. A 14" dragon can be anywhere between 4 months old to a year.
It is best to feed insects once a day, all it can eat in 10-15 minutes. Mixed greens (like the kind you are offering right now) should be available all day long (change after they start to dry out). Dust crickets with a good multi-vitamin and calcium with D3 once every other day. Feeding the crickets dusted food isn't enough though it's a good practice.
Cage size should be at least 4'x2'x2' and you don't have to buy a large glass tank. You can build a tank for alot less than a glass tank will cost. I built a 5'x2'x2' tank with wood, screen and plexiglass for about $160 CDN. For the back, bottom and half the top I used one sheet of plywood underlay 1/8" thick. for the other half of the top, 1/4" hardware cloth. The front was aluminum screen and the sides plexi glass. This is just a basic cage with a 4' double florescent fixture and ceramic fixture for the basking light. Except for the UVB tube all the equipment can be bought much cheaper at a hardware store.
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PHLdyPayne