We put our older one in PreK when he was 2.5 We wanted him to start at 3, but when were researching preschools in the middle of the school year we found one we liked and decided to enroll DS right away.
All went well at the beginning. Everything was new, he was all excited, he didn't even want to go home. The honeymoon took only a few weeks. Things went downhill from there. Then he was home sick, then back in PreK, then winter break, back to PreK ... He no longer wanted to go. The novelty had worn off, he was bored, he wanted to be home with me and his brother. He cried, we tried different things, but at the end we withdrew him from there and put him back when he was 3. He was completely fine at 3, but it was a disaster at 2.5. With our younger one we waited till he was 3.
Even if everything seems to go ok at the beginning you may still face problems later on. You may want to ask your child what he thinks from the beginning, but it may be better to phrase it as "What did you like today?" "What was your favorite part?" ...
Like others said two year olds don't interact too much. Your child won't miss anything if he doesn't go to PreK till he is 3 or even 4.
