There is no clear cut definition to old age or no set boundary to it either. Back then quite a lot of people use to say that when someone has grey hair they are considered old but scientifically this is not true because even kids nowadays have grey hair due to genetics. But in general old age means when yo become weak and then start to depend one someone for assistance with their day to day activities which they were able to carry out alone when they were young.

A lot of things change when you become old physiologically, physically, mentally and psychologically. When you consider the physiological changes one can notice wrinkling of skin and maybe one can lose weight. Even crouching of ones spine could take place. But the hair becoming grey is related to genetics but it is seen in old aged people and their teeth fall off. Physically they are unable to perform tasks like carrying furniture or even items heavier than one kilo so they may need an in home care. And sometimes their blood glucose, cholesterol and blood pressure might start to change depending on their life style and these changes might come drastically.

Mentally their brain capacity to remember things starts to decrease and sometimes even their memory of the past can decrease. In this case they will start to forget to take their medicines or might fail to remember to place a monthly doctors check-up so retirement home in Sydney will cater to all these loop holes created to due old age. They might start to mix up stuff and start mumbling to themselves. All these happen due to old age. When it comes to their psychology they tend to be needier. They would demand for stuff and also for attention. In literature Shakespeare compares an old aged person to a baby because they tend to seek assistance and psychologically they want everyone to talk to them and hear them out.

They might fall prey to mental disorders like depression and insomnia. Some old aged people do not consider them old and might want to try extreme things such as going on a roller coaster and so on. This is mainly because they do not want to feel inferior an out-dated.
Reaching old age is an experience in itself that no one can explain till they become old themselves. The experience varies from person to person depending on their genetics and lifestyle. But the main principle changes are pretty much the same.