I understand English, but I cant speak it. Why?

You have started learning English without using it. Once you make E  part of your life, you will see the results. You need two things to really grasp a language- motivation to practice and speaking with others

Am I too old to learn?

It is never too late to do what you want. Well, it ultimately comes to the individual's dedication level and willingness to put in the time to practice. It is actually beneficial for your brain-like workout. The most important is your attitude and goal. Do not get discouraged by others. You are doing it for yourself.

Feeling nervous when speaking-thinking too much about grammar. What should I do?

Forget about grammar, concentrate on being understood. There is no use of having good grammar if you can't communicate. Don't worry, what people think or if you look silly. Most people will try to help you or encourage you. Making mistakes leads to improvement. try to use language as much as possible-speak about something that excites you. You can speak to yourself to get some confidence first,,,