Use 'mountaineer' to describe someone actively involved in climbing, not just interested in mountains. He became a mountaineer after years of training.
Pro Tip 2/3
Not Just Any Hill
'Mountaineer' implies climbing significant mountains, not smaller hills or slopes. As mountaineers, they dreamt of conquering the high summits.
Pro Tip 3/3
Professional Implication
A 'professional mountaineer' does this activity as a career, which includes sponsorships or guiding others. She earns her living as a professional mountaineer.