18个月. But you need to add (大)的 after 月: 18个月(大)的宝宝. 5个月 = 5 months 5月 = May So 个 is absolutely necessary in 18个月. Some people are so lazy that they omit some words in writing. This rarely happens in speech, though.

18个月好一些。但是如果你说18月，大家也能懂

I am certain that 18个月的宝宝 is absolutely natural way to say "a 18-month-old baby" in Chinese, native speakers don't say 18月宝宝，this is an incorrect expression

18个月大的宝宝 the most correct answer

measure word is a must