I'd suggest a 12 month beardie. Its basically in between a juvenile and an adult until it reaches 2 years of age and becomes an adult. Basically a yearling or sub adult dragon is what it is. This way you can still watch it grow up just a tad but it wont be as fragile as a juvenile or baby. And it also wont eat as much as a younger beardie.
Actually some even live up to 20 years, 15-20 is the average with good and correct care.
I got my two eldest beardies when I was 13, both were around 6 months at the time I got them. They're both two years of age now, and before my 14th birthday I got another beardie in July and now shes about 13 months of age (I got her when she was 4 months old).
They make wonderful pets, but definitely not beginner pets... not because of how long they live but because of how much they cost and how much time you need to put into them.
Make sure you can provide at least a 75 gallon enclosure as that is the minimum, 75 is a bit of a large enclosure and some people dont have the space for it. Make sure you do tons of research as well.
https://docs.google.com/document/d/18PfjxbgD95KLcfRCqGAVz-ViuMPuP4AvgGBSZzPKRgc/edit?usp=sharing
I made this
care sheet on them if you want to look at it, it basically explains how to take care of them although I left out burmation and breeding for now.
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