multiplayer and demo problems

I have recently migrated to a Microsoft account then when I launched Minecraft it wouldn't let me play multiplayer, then today when I opened the Minecraft launcher I made me re-log into my Microsoft Minecraft account then when I launched Minecraft it said that it said I didn't own Minecraft and would only let me play the demo. Also I might not be able to play multiplayer because my Microsoft account is under 18? is that the reason.