There's no single minimum age for volunteering at a preschool. It varies widely depending on the specific preschool's policies, the nature of the volunteer work, and local laws (some jurisdictions may have age restrictions for unsupervised work with children).
Some preschools might accept volunteers as young as 14 or 16 with adult supervision, while others may require volunteers to be 18 or older. Always check with the individual preschool you're interested in volunteering at to find out their specific requirements.