Boy this is one I was VERY hesitant to touch.
Stiff deads can be, as with any movement, dangerous if form is not correct. But I love them myself and feelt thewy are a very effective movemnet that has a place in everyones routine. It is simply impossible to tell how you are doing with the loads you have now over the internet.
The second being your age. You are still growing etc… Now is no time, nor is any other, to mess up a major part your body like your back due to piss poor form from lifting higher loads with ego, instead of lighter loads correctly.
I would say stick close to the reps you have. Maybe drop to a 5x5 scheme with a little higher weight. Make sure you ALWAYS use safe form. Try and progress on adding a rep or two each week to the total of ALL work sets. Once you are able to move the 5x5 load for say 5x7-8 move up a little and start over.
Also just mix it up. even if you mobe to some lower rep higher load work dont forget about the higher rep stuff forever. Go back to it as well from time to time. Or even go to a 10x3 where you use say that same 5x5 load but are stopping well short of failure. Getting 10 very clean crisp sets.
In the short, mix it up, have fun, dont over anylize this, go lift, above all do it safely and enjoy every minute.
Hope that helps