Sago's and their counterparts in the cycad family have very sturdy, almost plastic like leaves. They are closely related to the ancient plants that lived on earth during prehistoric times. Their growth comes out from the center like other palms. Growth is very slow.
The lower and older fronds usually yellow and die off as the plant ages. Most palms only support a certain number of fronds. When they reach their quota, a new one replaces an old discolored one.