I'm not sure I understand your question...in the title, I think you are questioning whether folks apply for medical school during junior or senior year of college? To answer that question, it is always (beginning to mid) "senior year" or summer directly before it, seeing as almost every med school requires you to have obtained an undergraduate degree before matriculating. Most students, as of their third/junior year, have an entire extra year or at least 1 more semester left to complete all credits required for their degree, thus, they can only apply senior year. There are rare cases, though (I knew someone, in fact) where you finish college an entire year early...so while it is their third year in college and technically junior year, they're still credit-wise qualified seniors.......hope that made sense....it did in my head.
As for the question within your post, I think you are asking: do more non traditionals apply or do folks take a year off before matriculating?...(not sure what "take the lag year" quite means...)
To answer my interpretation of your question, yes, there are tons of non-trads that do matriculate, esp in DO schools. I, however, am a traditional, but have taken a year off in between undergrad and matriculation, making me 24 when I start (I was an 08-09 app cycle alternate list candidate, btw).
Hope this helped!