I am 39, am currently not doing any formal exercise (just walking my dogs at a slow pace for 45 min. - 1 hour at a day), and I can lose 1 lb. a week on 1500 calories a day. I'm currently 156. I'm hoping to get down to 135, and I'm sure before I get that low, I'll need to start exercising.
The following website gives a detailed calculation to determine your daily calorie needs to maintain:
http://health.ivillage.com/eating/en...,49h-3,00.html
(This links to page 2 of the article, but page three contains essential information as well).
I like this equation because it takes into account how much your metabolism decreases as you age. It's the most detailed calorie calculation that I've found on the web.
Of course, once you find out your daily calories needs to maintain your weight, you'd need to deduct 500 calories per day to lose 1 lb. a week. An alternate way of calculating your calories would be to just put in your goal weight and see how many calories it would take you to maintain your goal weight. If you stick to THAT number of calories, you should eventually get down to your goal weight.