A brilliant question and one that is keeping people bussy the world over. I shall try to give some sort of an answer.
The cells in your body are forever dividing to provide you with new skin, liver etc. (although some such as nerves never regrow). It is thought that as this process proceeds certain protines are passed on in smaller and smaller amounts, but as these are essencial for cell division and keeping you healthy, the tissue of your body becomes renewed less and less efficiently. Elasticily in skin is lost (hence wrinkles) skin becomes thin and papery. Muscles become slack and so on. Some things will cause this process to proceed quicker, ie smoking, some things may help stave off aspects, ie excersising muscles to keep them tight. But in the end your body will slowly slide into decline.
What a depressing thing to be writing about on a Monday moring! I hope this helps.