We start with around 100 billion, but in our 20s this number starts to decline.
By 40, we could be losing up to 10,000 per day, affecting memory, co-ordination and brain function.
Bladder capacity in an older adult generally is about half that of a younger person - about two cups in a 30-year-old and one cup in a 70-year-old.
This causes more frequent trips to the loo, particularly as poor muscle tone means the bladder may not fully empty. Start ageing at 20 Lung capacity slowly starts to decrease from the age of 20.
Their job is to ensure the flow of information from one cell to another, and as we age we make fewer.
French doctors have found that the quality of sperm starts to deteriorate by 35, so that by the time a man is 45 a third of pregnancies end in miscarriage.
In fact, while the neurons are important, it's actually the deterioration of the gaps between the brain cells that has the biggest impact, says Dr Wojtek Rakowicz, a consultant neurologist at Imperial College Healthcare NHS Trust in London.
We all assume grey hair and wrinkles are the first signs of ageing, but some parts of your body are worn out long before you look old These tiny gaps between the end of one brain nerve cell and another are called synapses.
The bladder starts to contract suddenly, even when it's not full.
Women are more vulnerable to bladder problems as, after the menopause, declining oestrogen levels make tissues in the urethra - the tube through which urine passes - thinner and weaker, reducing bladder support.
Constipation is more likely as we age, as the flow of digestive juices from the stomach, liver, pancreas and small intestine slows down.