You do not know how much I have wanted to see you. I am so glad that you decided to come to Istanbul. Can I say to a person who is 20 years old: (You've really grown up./You've become such a gentleman./You are really handsome.) [Any of these is acceptable. It just depends on what you want to say.] What else can I say to him when I meet him for the first time? How can I compliment him? I've only see him in the picture when he was little. - You could comment about how he has changed in terms of his appearance. E.g. You're hair looks a lot different! I always keep his picture in front of my mirror in my room. At the end of the conversation: can I say I will hang out with you or come over to my place. I hope you enjoy the time here ( because he is a visitor to Istanbul) I will see you soon. - This depends on what you're intention is. Hanging out suggests to just go out and be with the person somewhere in a public place. Come over to my place suggests that it's an invitation to come to your home/house. Hope this helps.

OK. sentence :)