I have a website that was created five months ago, but I hadn't optimized it that time. Now I plan to optimize its meta title and description, is it bad to my site?
It's never too late to optimize. Assuming you optimize ethically, your site's position will rise in search results and your traffic will increase. And that's a good thing!
No its ok, do it properly. Place all metas correctly with proper header, footer. Inter linking should be done well.