I've self harmed when I was younger and am wondering if that would cause me to be unable to join the army.

Hi, if this is recorded on your medical records then the answer may be a straight, no. Unfortunately if you were harming when you were younger, the pressures of army life could cause you to harm again and the army has a duty of care to uphold. Putting someone with a history of self harming into the army could cause a trigger than you begin to self harm again.
If you have nothing on your medical records about self harming and have not self harmed in a number of years and genuinely believe you can take on the rigors of army training, the pressure of army life (potentially moving away from friends and family) then give it a go. There are a number of people who've self harmed when they were younger currently serving! Good luck.