The maximum bound for most gTLD registries is ten years (have a dig around the ICANN website if you'd like to check), not fifteen. Sure, there might be a ccTLD that allows you to register a domain for longer than that, but I'm not aware of any, and ccTLDs usually have lower maximum renewal/registration bounds than the gTLDs.

If a registrar is letting you renew a gTLD for 15 years, they're basically sitting on a third of your money until the domain's expiration date comes up.