Virgil is quite literally Patton's son. And Janus'. (And more things to think about/incorporate into fics if you want.)
Virgil really only exists because of Janus and Patton. Virgil embodies anxiety. The sense of anxiety is created by two things. The thought of what could happen to you (stemming from self-preservation, or Janus), and what could happen to other people and the world around you (stemming from empathy, based on morality, or Patton). When you think about it, Janus is the main parent, but it took both of them to create Virgil.
Furthermore, Logan is the reason Janus is fully formed. Logan represents the knowledge Thomas has gained, as well as logic. Janus relies heavily on the knowledge and logic Thomas has to know how to best protect Thomas. So Logan isn't exactly Janus' father, because all humans have innate self-preservation to a degree, but Logan is definitely the reason Janus is so developed. Like an older brother.
Because logically speaking, Logan is the oldest. He isn't an evolutionary trait like Janus is (predators and dangerous environments causing the need for instinct). He basically comes free with having a brain.
Roman is also likely a little older than Remus, because while they embody the same trait, they are made up of different thoughts. And the thoughts that make up Roman tend to come first. They were also dependent on Logan for their growth. pretty much everyone was.
Ages from oldest to youngest are likely as follows: Logan, Janus, Roman, Patton, Remus, Virgil.
The reason Patton is placed in between the twins is because they were only separated once morality came into play, though I don't know if Patton would be older than Roman.
If you've read this far, thanks for reading my silly rants and rambles. I like ages and things.