I've done bassinets on AC and UA with my kids, and 10 months old is definitely too old for the bassinet. Rule of thumb that one FA told me was between 6 to 8 months, depending on the development of the child, and that she has refused children older than that due to the safety issue.
If you ever want a bassinet, call ahead, right after you book your ticket. Do not wait until the gate, as most likely those seats have been taken or other families may have requested the bassinet. As others mentioned they are only in certain rows in Y and J, but if you call in, you have priority on them.
My first trip was at 2 months from YYZ-GRU, and I wish I had done my research about bassinets! I was in J, and ended up just sleeping in a pod with my baby beside me.
They will be installed once you've reached cruising altitude, and have a mesh over the top you're supposed to snap down to cover your child for safety (same as us being told to wear seatbelts when seated), which is why a child who can sit up is too old. If there is turbulence, you'll probably be asked to pick up your child and hold them.
Ad for timing, there is no perfect timing! Every child is different, and what works for one parent may not work for another. At 2 months my child slept 90% of the 11 hour trek to Brazil. At 14 months to Europe we one again booked a red-eye, leaving YUL at 8pm. Big mistake - he was so excited by all the planes, flying, noise, etc., that he actually didn't sleep on the plane.
At one point it was indicated by an FA that there was a nice open area at the back of the plane to walk around and soothe him, which was a "get your child out of business class" message. My wife and I arrived in Zurich destroyed, but thankfully our family there took him while we slept. We swore we'd never fly again... but we have many times, with mixed results.