I had these spots as long as I lived my father had the same issue
Hello,
These dark spots on the lower lip can be due to use of cosmetics, sun exposure, smoking, dry lips etc.Rare but serious causes of dark spots on lips are oral melnaotic macules and Peutz Jegher ‘s syndrome .Keep your lips well hydrated. Apply lemon juice mixed with glycerine.Cutting down on tea, coffee and quitting smoking may help. If it persists consult a dermatologist. He may suggest appropriate treatment. I hope it helps.
Take care and regards!