I'm speaking of my own personal experience. It's not like you miss your first love, ur just feeling the nostalgia (since it's literally your first time loving someone, it's like riding a bike, once u learn how to ride one you never forget, but you will always remember the first time you tried...dk if it makes sense lol) 

But, for my personal experience again, if my ex-partner get into a relationship right after breaking up it defo sus, and it certainly means that you were never two in the relationship...

For the first love topic, I think t depends on everyone, you can meet them early in your life like being a teenager and all, or later, it really depends...hope this helped tho lol