The Verdict: Need More Info
You can apply once you turn 18, the minimum age in all six of Spain's agreements. Use the time to find out what your consulate asks for.
The age is tested on the day you submit the application, not the day you travel, so you can prepare everything in advance and apply on or after your 18th birthday.
The slow part is usually health insurance, which must match the Spanish national health system with no co-payments or waiting periods. The upper age limit is 30 for most countries and 35 for Canadians, so there is no rush.