"Thank you for Joining Samsung community. We would recommend taking some troubleshooting steps to further isolate what you may be experiencing. Find troubleshooting steps on provided link http://www.samsung.com/in/support/skp/faq/1119803. Please let the community know what steps you have taken and if you have noticed any changes or improvements. This will help the Community to provide you steps without repeating any steps you may have already taken. "
This is quite a common problem on most devices not only Samsung although it seems to be accelerated on Samsung devices. Overheating occurs due to several reasons. 1. Processor use at high clock speeds. 2. High screen brightness. 3. Poor data connectivity. Since you mentioned YouTube and other online apps I am inclined that Problem 2 and 3 are the cause to your issue.
Don't worry about using your device when it is overheating as it will not create any problem. Although in the long run and in day to day use there might be drop in battery performance.
