I recently started riding again myself and found that I couldn't stand immediately either. I don't have any problems with balance or things that you would normally attribute to feeling unsure standing. After the first week or so I hadn't stood to ride at all and was at a light waiting to turn left to get the the LBS when the green caught me off guard. Sure enough my childhood reflexes kicked in and I stood up to stay ahead of the traffic that was moving past me. Turns out it was my weak calf muscles and my week of riding had strengthened them enough for me to be able to stand.
I weigh in at about 280 and am a young guy, so don't be discouraged at the time difference. Everything comes in it's own time and everyone has a different pace.