A harness suitable for a month old hatchling who might be maybe 14g and as small as 5g will be totally inadequate for a big STRONG fully grown adult beardie who might be 700g or heavier.
I wouldn't recommend letting a very small hatchling have much if any freerange time so I don't think a harness is suitable for one.
It's a different suitable with a 6 months old or older beardie who will be considerably bigger and will enjoy times out much more, but getting a one size fits all harness is not going to be practical.
I found with little hatchlings that the slight pressure / weight of the wrist my left hand on the skinny end of it's tail while it was sitting on my tummy/chest , was enough to discourage the little ones from suddenly leaping / dashing off.
Also if you on the ball, and see the little one wanting to crawl, dash off, it's very easy to simply place the free right hand infront of them to "head them off".
