a blood pressure of 109/48 and a heart rate
of 65 are both healthy and normal for an individual who is either athletic or work out a lot. You said you're 15, so, if you are athletic and work out a lot, then you have nothing to worry about. If you're not, a blood pressure that's constantly below 90/60 is considered to be low blood pressure and should be checked out by a doctor. Also, it's better if you have someone in your family take your blood pressure manually (with a cuff, aneroid meter, and a stethoscope) rather than with one of those monitors because they don't give a perfect reading. You can get the whole package at Walgreen's for under $20.
If you need more info. on how to take it manually visit