Agree with tallulah. I bf my 18 m.o. all over the place out & about, and either people don't notice, or look at him but don't realise what I'm doing. Those who do realise just tactfully look the other way.
The only person who will notice will be the person sat next to you. It's actually quite hard to get a really good view of the other passengers on a plane, even if you want to. [nosey emoticon] I feed my 17 month DS everywhere and have never had a comment or dirty look yet. I'm almost disappointed!
Done it myself with DS at about that age years ago and another smaller one more recently. Had comments afterwards about how well behaved my children were on the last flight (we were stuck on the runway for 5 hours with no AC or food ). Go for it.