The term can apply to anyone keen on mental activities, not just professions like scholars or philosophers. Even though he never went to college, his constant questioning shows he's a true intellectual.
Pro Tip 2/3
Respectful Tone
Using 'intellectual' suggests a tone of respect towards the person's mental capabilities or interests. My aunt, an intellectual, is respected for her insightful views on art history.
Pro Tip 3/3
Interests Matter
Being an intellectual often involves interest in a variety of subjects and a lifelong pursuit of knowledge. She could be considered an intellectual because she spends her free time mastering new languages.