You need to replace your helmet if it has been involved in a crash. If it's been properly taken care of and hasn't been involved in a crash or another impact, it should be replaced approximately once every five years, or every three years if used regularly. Helmets are made of materials which will deteriorate with time, heat and humidity. Hair oils, body fluids and cosmetics also contribute to helmet degradation. Petroleum-based products present in cleaners, paints, fuels and other common materials may also degrade materials used in many helmets, possibly degrading performance.
It is recommended that you don't buy a helmet online. Good fit is essential to optimal head protection. There is no way to know whether a helmet will fit well based only on your head size. People with the same head size may have different head shapes. Without trying on the helmet, there is no good way to tell.
Generally the answer is probably not. Frequently dropping a helmet on the ground, or other hard surfaces may degrade the helmet's performance. However, if the helmet falls to the ground at highway speeds when not being worn, you should be aware that some degradation may have occurred. In general, the real damage comes when the helmet contacts an object with a head inside. It is recommended that if you suspect your helmet may be compromised, then replace it. If the helmet has been involved in a crash while in use, replace it immediately. Never re-use a helmet that has been involved in a crash.
Firstly, you should try a few helmet models to find one that feels most snug and comfortable all the way around the head. To check if the helmet is too big, you should buckle the strap and try to pull the lower back of the helmet forward and then push the front brow area of the helmet backward to see if the helmet will slip off either way. If it does, the helmet is too large. To make sure the helmet is not too small, you should leave the helmet on your head for at least five to ten minutes to see if there is any feeling of pressure point. Some helmet models have exchangeable cheek pads for better comfort. Also have a read of the Helmet fit section.
Never buy a used helmet or borrow someone else's helmet. Over time the protective foam in helmets will adjust to the contours of a user's head. A used or borrowed helmet may not offer as much protection as a new helmet. It is impossible to tell by visual inspection alone whether or not the helmet has been involved in a crash and whether its protective function is significantly degraded.