Age gap!!

Iam 31 and my partner is 23. We met 4 years ago. It's only just recently that I have noticed the age gap is a problem now. Because I want to get married and he doesnt want to talk about it. When is the ideal time to get married after 4 years together? Longer or shorter? Iam paranoid that I'm getting older. What age do you think he will be ready? Or should I just hold back and. When is ever the right time?