This Mammillaria formosa is relatively young for me. I've only had it since 2015.Sometimes the Forum software turns my pictures sideways. Sometimes it doesn't. And I can't figure out why or do anything to control it. Sorry people.
2015
This first picture was taken on 12/27/2015 with a quarter in the pot for scale. Since an American quarter is 23mm across, I can measure the cactus in the picture and do a little math to tell you that the cactus was 3 1/8th inches (just shy of 80mm) tall in December of 2015. 2016
Again, in December ... but a different year. Without a coin for scale, so I can't calculate a size. It flowered for the first time this year. 2017
And started doing a fair job at producing flowers in Apr of 2017. Again, no coin for scale. And by November, they were mostly gone. 2018
Except for the next picture here, these last five pictures were taken today.
It started flowering again in March. From the side. It's 6 1/8 in tall ... in MAY ... and the growing season has just gotten good and started. From the top And REALLY close to the top. You should be able to see the dish shape growth on the top. And really close to the side to show spine structure (in case you couldn't see it in all of the other pictures).