Keng Leong Yeo
Do I have to place the barn only onto the legitimate place created in my current turn (i.e. one of the four connecting meadow tiles must be just placed), or I am allowed to place the barn onto any eligible available spot (incl. those created earlier in the game)?
You may only place the barn on the legitimate location you created on your turn and not anywhere else.
Quoting the rules (page 11 of my Big Box 2 rules):
When a player places a tile that creates a junction of four tiles where each of the four tiles has only a field segment at the junction, the player may place his barn instead of a follower. He places his barn at the junction of the four tiles.