I Am In 50s. Need A Girl Friend Who Is Young,can You Help?

The easiest way is to get registered with websites that offer friendship matches in your area. There are plenty of websites like that in which you can create your profile with your interests and get registered with it(sometimes after some verification). You can also search others matching your reqirements. Since I dont know your country, I can't know websites which offer good friendship opportunities in your country.
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You can also use chat rooms on different websites related to your country.
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If your GSM company provides any friendship service, then use that opportunity too. It is one of the most easiest and efficient one.
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Another way is to look for your local clubs which provides opportunity of meeting with new an also use chatpeople. You can also find there someone for you.

Your question is a difficult one to answer as most young girls who are prepared to provide excitement and thrills for a man in his 50's will not be the kind of girl you are looking for. If you want someone from a 'decent' background, you have to put up with what comes with it (in India) ie no privacy with your girlfriend until you promise marriage.
I don't mean to be intrusive or bad-mannered, but how come you are not married already? And if you are already married, this is not the appropriate kind of website to be looking for extra-marital thrills with young girls.