The universe is about 13.8 billion years old (Planck 2018: 13.787 billion, give or take 0.020 billion). Light can only reach us from places that were close enough for it to arrive in that time. Anything beyond that is out of sight, simply because its light has not got here yet. The sphere of everything we can in principle see is the observable universe. We sit at its centre, because it is defined by what reaches us.

Is it 13.8 billion light-years in radius? Almost, but no. It is about 46.5 billion light-years in radius (so about 93 billion light-years across).