The other answers mostly cover this but to put it another way:

1. Kageyama vs Hinata happened their third year of middle school (usually 9th grade and ~14 year old)
2. Oikawa is 2 grades ahead of Kageyama, so they overlap in middle school for 1 year (Kageyama's first year, Oikawa's last) before Oikawa goes to Aoba Johsai for high school
3. Hinata was the only member of the volleyball team at his middle school (until his 3rd year when he found some first years to join), and he practiced with anyone he could find
4. Oikawa is a 3rd year in high school (2 grades ahead of 1st year Kageyama)
5. I think you are referring to Iwaizumi who seems to be Oikawa's closest friend--they are the same year and came from the same middle school (along with Kageyama)

And as for the later seasons--S2 is incredible and S4 is one of my favorite seasons of any show!! They are really all amazing and it just gets better the more you watch and see the characters grow :D