My macbook pro has been showing this for something and it has been like 3 years since i last changed the original battery. For those people who just noticed this in your osx battery status, please quickly go and purchase a new battery to swap out yourself. You may go to any Apple Authorised Shop to do it too, of course it will be more costly.
Clicking on the Service Alert will bring you to the screen as above.
I also installed this App from Apple App Store called Battery Health. It is able to monitor the number of cycles, current battery health and age of the battery.
Although i swapped out the battery a couple of years ago, it still shows an inaccurate age. I guess the information is embedded in the battery as the battery was manufactured in 2009 (the year i bought my macbook). Looking at the current max, my macbook's battery has about 3/4 life left. But sometimes i do experience sudden shutdown when the battery still has enough energy.
I have ordered a replacement battery from Aliexpress and this is the supplier. Apparently seems to be from Hong Kong and he only sent out the stuff on the very last day before the order expires. Hopefully my new battery arrives soon and i will post the update and replacement video once i receive it.