Michael this was taken from the link that Dedworth provided in his previous post. I was also curious about the age limit, as they seem to be stricter here in the Philippines.
Most people can give blood. As long as you are fit and healthy, weigh over 7 stone 12 lbs (50kg) and are aged between 17 and 66 (up to 70 if you have given blood before) you should be able to give blood. If you are over 70, you need to have given blood in the last two years to continue donating. However, If you are female, aged under 20 years old and weigh under 65kg (10st 3lb) and are under 168cm (5' 6") in height, we need to estimate your blood volume before donating
http://www.blood.co.uk/giving-blood/who-can-give-blood/