The reading し is used in compounds and formal contexts; the standalone word for 'tooth' is usually は.
See also: 歯 (は)
A dentist treats cavities.
Straighten the teeth alignment.
Used in formal expressions for age, often in set phrases like 歯に合う (to be age-appropriate) or 歯を重ねる (to grow older).
He wears clothes that are too young for his age.
As one grows older, one gains wisdom.
molar; grinder; premolar
tooth extraction; pulling a tooth
dental calculus; tartar
baby tooth; milk tooth; deciduous tooth
toothache
tooth root; root of a tooth
canine tooth; eyetooth; cuspid
gums; gingiva
teeth · teeth and tusks
artificial tooth; denture
saw teeth; serration · leaf serration
alveolus; tooth socket
dentistry
dental pulp; tooth pulp
permanent tooth
carious tooth; decayed tooth
false tooth; temporary tooth
incisor; front tooth · grinding one's teeth; bruxism; gnashing of teeth
teeth clipping; teeth filing
zipper teeth; zip teeth
lips and teeth · mutually dependent relationship
arranging seats by seniority
tooth replacement; replacement of lost teeth
teeth clipping; grinding down teeth
crown of a tooth
The kanji 歯 means 'tooth'. The reading し is the on'yomi, used in compounds and formal contexts. The sense 'age' is a metaphorical extension, as teeth are associated with growth and aging.