It will PROBABLY be fine but I put my then-7 1/2 year old (55th percentile for height, 50th for weight, ish, 45 lbs. at the time, very small bones/frame, and willowy, lots of height in her legs rather than torso) in a seatbelt only on a couple American flights and the belt would not tighten enough on her.
Unfortunately we had no seat for her and had to just deal with it. She was scared and uncomfortable because it was loose the whole time. It all has to do with frame, I think, as well as the seatbelts- she'd probably have been fine on some airlines/airplanes where the belts tighten more. The different planes have different belts so it's only an issue on some.