Those with 'only' children now TTC #2... How old is your first?

My son will be 6 this May. For various reasons my husband and I only feel ready to try for a brother or sister for him now (and dh needed some convincing as he is in his forties). I'm just wondering if anyone else will have a big gap if they have another baby? My son is the one who keeps asking for a sibling! He would be a great big brother, so I'm keeping my fingers crossed.