How long until they crawled on all fours?

My older daughter never army crawled. She just crawled across the room out of nowhere at 7 months. My younger son is 7 months now and can army crawl quite well. But I've never once seen him on all fours. I'm not worried per se, he can get where he needs to go, sits up independently for long stretches, and is only 7 months anyway. I'm mostly just curious if and when I can expect him to real crawl. He loves to hold on to something and stand up so part of me thinks he might go straight to walking.