Adults loose teeth primarily through accidents or disease. The
former are pretty clear, if you get hit in the mouth by a hard object
your teeth may get knocked out. I assume you want to know about the
latter.
Teeth are complex structures with blood circulation, nerves, and bone
all beneath the white enamel covering. While the enamel is tough, the
rest of these parts can be damaged easily. Gum disease is a big
factor. If the gums recede from the enamel, it exposes the underlying bone
and allows infections to start destroying the teeth. Gum inflammation can
lead eventually to receding gums.
This is different from why young children loose and replace teeth.
I hope this helps.
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