Is it normal that my 7-month-old hasn't smiled yet?

No. Babies normally start to smile at 2 months old. A 7 month old who does not smile is at risk for developmental issues.
No. Almost all babies will have smiled responsively by 3 months of age. If your 7-month-old is not smiling, this suggests a delay in social development and you should consult your doctor right away for further assessment.