The Obsidian Airflow bike helmet is the best all-around option for teens and adults for under $100. This helmet is a great option for commuting and every day use for short/medium distance rides. It has 22 vents to keep your head cool, it can be adjusted both horizontally and vertically for a snug fit, and it has a lifetime manufacturer warranty.
Con Too tight for larger heads, and too large for toddlers
This helmet might be too tight for people with a larger-than-average head, as this helmet comes in two sizes only and there's only so far you can go with the adjustable dial. The sizing starts at around 50cm in diameter, so it will definitely be too big for toddlers.