Is a IQ of 140 for a 15 year old good?

It doesn't matter your age when it comes to IQ scores. You are scored compared to other people in your age range. 140 would be considered a good score but if it is just an online IQ test, I might doubt the accuracy of it. The average score is 100. I'm not sure if you know about standard deviation but for every 15 points over 100, that is one deviation. So, 140 would be significantly higher than average.