Ouch, sorry, I have a bit of a pet peeve when it comes to "how long should I wait before..." questions in relationships. Honestly, there's no blueprint. If you want to text him, text him. There's no need to wait. What you say and how you say it will mean a lot more than the timeframe, so if you use wording that sounds needy and desperate it won't matter if you wait weeks or months. At the same time, if you sound confident but just wanting to spend more time together, then it doesn't matter if he hears from you immediately after you see him.

Sorry about the rant. On to age: 27 and 38 is a good stretch, but you gotta make your choice over time. That's the key part. The age difference is nothing is you two really get along well, communicate great, and enjoy being together. Others may have a problem, but you'll never make everyone happy. My grandparents are at least 9 years apart, I know that. But the over time part is key. Constantly assessing over time whether this person is someone you want to spend the rest of your life with is the most important part. Just be honest with yourself and him, and everything will be fine.