Hello and thank you for your question.
Any time you have a red lesion on the side of the tongue that persists longer than a few weeks it is important to have it evaluated in person by a dentist
and possibly biopsied. Oral cancer
can occur at any age and is very common on the side of the tongue. The tried and true method of identification is through biospy. Many lesions can appear flat, red and on the side of the tongue.
Highly unlikely that it is cancer at your age, but safe to rule out if it doesn't go away. Geographical tongue does have a border but not always apparent. It can also be lichen planus
, trauma from an adjacent tooth, chemical burn
from medication, new toothpaste, mouthrinse or spicy/citric foods.
I hope this helps you.
Best wishes, good health.