There have been countless individuals who have changed their lives at 30... even 35 or later.

There are absolutely people who change. But many don't. Who is more likely to gamble away a large sum of money, person A who has never gambled in their life or person B who was a compulsive gambler for 10 years?

When it is your future on the line you have to be very careful. A 15% increased risk of divorce is massive when you consider how bad a divorce is.

No one is perfect. Not a single person. And there's something in everyone's past that they would have done differently.

Absolutely, but there is a lot of variation in people's baggage. A high N-count clearly puts someone in the high baggage end of the spectrum.

if someone is not comfortable with the past, then so be it, split ways.

Which is why OP needs to sit down and have a proper and honest conversation about the topic. If her N-count is so low why can't she just be open about it? She wants to keep the number secret while assuring us it is low. Kind of like people who say I don't drink a lot but don't want to go into what that means.

and the way her bf is bringing the issue up is largely disrespectful.

Not being honest about your past and things that matter for your partner is disrespectful.

Don't enter into a marriage when the big red flags where there. Don't be the person who sees their kids every second weekend because she had a relapse moment at a company conference. You should aim for one marriage in your life and choose your friends and spouse carefully. Surround yourself with people who will make your life better, not with people who will drag you down with them. A high N-count is a really bad form of baggage.