As I got through the chapters and took notes of Wikipedia, I noticed some problems with my age and the timeline, so this is to make things clear.

Timeline:

She is addressed as 'noona' by other characters, but she is really just 18, the same age as Daniel and most of the other characters.

Minseong is nine. It has been three years since she last met Johan when he was 15 at the time (Ae-ri was 15 too)

Samuel and Jake mentioned that they were older than Daniel, and according to Wikipedia, they are around 18-19.
(Let's just say they're 19)

According to Wikipedia, Gun and Goo are 19-20, but I believe Goo is a year older than Gun (20) even if it's unknown or not canon... It just feels right to me.

Since Gun mentioned in the early chapters that he was also taking the GED, I will assume he is 19.

Goo - 20
Gun - 19

DG is 21 or 22 since it was canonically mentioned in Lookism (He said he was Gun and Goo's senior)

Past and backstory timeline:

The four major crews take place in different regions,

(J High School is in the center - Seoul - I think?)

North: God Dog (Gangbuk)

South: Workers (Gangnam)

East: Hostel (Gangdong)

West: Big Deal (Gangseo)

Since Ae-ri has met Johan before, she has walked down the streets of Big Deal twice, actually.

Second: Where she met Johan

First: Her relations with Samuel, Jake and Sinu
(will be revealed in later chapters of this book)

Due to Ae-ri getting whatever jobs she can, she does travel around Korea.

In three years, she transferred schools thrice then dropped out a month before she started working at HNH Company.

Her first few years were going to a public school before transferring to an all girls private school.
(She meets Sally and Heather)

She transferred to an all-girls school because teachers noticed she doesn't hit girls.

In that same academy but in a different class, she met Olly.
She is friends with them for three months, getting to know everyone else at Hostel in the third month.

She left before Hostel fell apart, transferring schools, going to an Elite private school.
(She had more money at the time.)

At the time, according to the manhwa, Goo comes over to Elite private school after helping Vasco and his cousin.

For another request, he helps Eugene. (Aged 15 at the time, Yuseong too.)

Since Goo wears the same clothes in his fight with Gun during Gun's past arc, I will assume that he met Gun near the same time with all those jobs and school.

Perhaps Goo met Gun before, since when he met Eugene, he even introduced Eugene to Charles.

The current story takes place in September since Daniel had Thanksgiving (November?) in chapters before God Dog, and I assume everything happened in a year and half.

So now it is September before Thanksgiving 2

Minseong's birthday is on the first of October!

That's about it :)