My question involves criminal law for the state of: Texas

I was present at a small party at a friends home. There was about 25 other people there; all but two where at least 18. I am actually 23 but did not provide any alcohol for the party.

The police arrived at the house on a noise complaint and came into the back yard (the gate was left open), by that time nearly everyone was in the house in a bedroom that was not visible from outside to the officers. There was one 18 year old who was outside the house and he was the home owner's son. I was the only person who was left in the living room, and the officer could see me through the window, which was fine because I had nothing to hide. The officer motioned for me to come outside and I did so, making sure that I closed the door behind me.

The officer asked me how old I was and looked at my ID and told me to just wait till he ran my license for any outstanding warrants. (I have none.) In the meantime he walked into the house even after the son expressed that he did not want the officer to enter the home. The officer then found everyone in the room and asked them to come out. When the son questioned the search, the officer said he had probable cause and told the son to "look it up."

The officer then charged everyone who was at least 18 but under 21 with an MIP. The officer claims that just because there was alcohol in the living room that it was readily available to the "kids" and therefor they committed an offense. They where not even in the same room as the cans of beer (which where visible through the large windows).

Two things perplex me:

1. Why where the two 16 year old's told they where free to go.

2. What probable cause did he have? Beer sitting on a table is not an offense, and no arrest was made, so what permitted the officer to come into the house against the son's wishes?

Thank you in advance for your responses, I am just a little confused about this.