Both my DSs were a bit over 14kg at 18 months. DS2 has just hit 15kg and is 22 months. They are both on the highest percentiles for height and weight though. Around 12kg is pretty average for 18 moths.

They definietly do slow down a lot after 12 months. After the rapid gains in the first 12 months it seems weird when you weigh them and they have only put a couple of hundred grams in months. It is nice to not have to put them in new clothes every month or 2 though!