Late to answer, and this isn't relevant to the post here, but to answer your question, yes. If they're both in the same grade, and if Natsuno's birthday was in December and if Megumi's were in August, Megumi would be older. It'd be unusual for someone who was 15 in December to be in the same class as someone who was 14 until the following August; a little more likely in most countries where the school years starts/ends in summer than it is in Japan, though still unusual.