My nine year old can program my cell phone, send text messages on his DSI, operate a computer to get to his favorite sites, download pictures from a digital camera, program the satelite receiver, and I would guess this is considered normal for a nine year old.
Isolation by height isn't going to work - I've seen three year olds drag over a chair to get on the counter
Keyswitch was brought up.
X-10 radio controller (keep the fob in your pocket)
Keypad (will only work until they see you do it one time)
Voice activated
Thumb print scanner
Retinal Scanner
Likely any casually dedicated teenager could get by these if they thought you were hiding something interesting.
cf