I started surfing 5 years ago. At age 58. It took a while to realize that there is very little help to get for old surfers. Most tuition and books is written for younger more flexible bodies.
The pop up, as you should do it, is a sure way to get a back ache.
I almost quit in despair. Then I remembered a mail in this forum that said. " Get onto your feet the best way you can "
After 4 years of desperate trying to make a proper pop up, I am using my knees every time now. Often I surf the first part of the wave on my knees and then get to my feet later.
Here is a video me surfing .
A study has been made by North American Journal of Sports Physical Therapy . Old people surfing.