Notwithstanding the fact that your child ceased being a baby about a year ago, there should be no issues flying up front. Ignore any thinly veiled arrogance of fellow passengers who unreasonably expect monastery-like sanctity in their precious J cabin - if your kid is well-behaved, he/she is welcome anywhere. The nice thing about an 18-month old is that (in my fortunate experience), they can understand and often succeed when you help them clear their ears to avoid pain upon descent - likely the main reason infants fuss & cry aboard aircraft.

Have a great flight.