Dave,
I have this also, it showed up at around the 19 year mark. From my own research and other message boards, hypo unawareness is caused by frequent hypos, where the body starts to act as thouugh it's a regular thing, therefore is doesn't release the warning signs (sweating, confusion, etc).
I used to start sweating profusely, even if it was mid-January, with any low going below 4 (72), but not anymore. Like you, I can be walking around at work in the 3 (54) range and not feel a thing. Last January, I passed out at work from a severe hypo, despite feeling fine 5 minutes before that - I was standing at the photocopier, and down I went - no time to do anything.
According to research findings, hypo unawareness CAN be reversed by avoiding ANY lows for a prolonged period of time. The main thing that I have done to combat this, is test a LOT more (I now test a minimum of ten times a day). While I still have frequent lows with my physical job, I can react quicker to avoid any ambulance trips to the hospital.