"more upgrades to choose from + gain 1 HP"Youth is an upgrade that heals 1 hp (or adds one to the health overflow bar beneath the health bar) and increases the number of upgrade options at the end of each level by 1.
Having more options for upgrades is very useful, especially taken early on. Improving your chances of getting a specific upgrade is valuable when you know the benefits of every upgrade.
Youth has a chance to appear as an upgrade after completing a level.
Boulder Style reduces the upgrade options by one, so Youth is effective to counter this.