This unfortunately isn't something we can change, we use Tebex to run our store and all we can do is add packages to their system, we can't make any modifications to their website. I've just had a look and there isn't any outstanding suggestions for an age verification or age gate, so you could submit one there? https://suggestions.tebex.io/ideas
SMPEarth was originally a private, whitelisted Minecraft server, featuring a custom-generated 1:3000 scale map of the Earth, exclusive to a group of selected popular Minecraft creators and streamers. Now, SMPEarth is publicly accessible on Java Edition where anyone can join. Boasting a community of 7000+ members and 2 main servers to choose from and experience (Casual Towny and Siege Towny), SMPEarth's primary goal is to become the most advanced Earth server in Minecraft.