A snug fit with no pressure points ensures comfort and correct position on the head when you crash. It may take a half hour of wearing to feel pressure points. Weight has not been an issue with today's thin shell helmets. Special Problems Pony tail ports improve fit and cooling for those with long hair. Bald riders can avoid helmets with big top vents to prevent tan lines.
Some head shapes require more fiddling with fitting pads and straps.
Extra small heads may need thick fitting pads. For a softer landing, seniors should pick a thicker, less dense model without huge vents. Prices and Where to Buy Check out this Consumer Reports article for some good brands. Then when you pick up a helmet, look first for a CPSC sticker inside and a smooth shell with a bright color outside. Put it on, adjust the pads and straps, and then try hard to tear it off.