SUP surf conditions are good up to 12 knots I find. My friend SUP while I surf. His website :
http://www.paddlesurf.ca/2012/08/photos ... t-jericho/He is the only SUP surfer, 100% into surfing, that I know here in Vancouver. When surf happens he is there and I am there, that's how we met, countless dawn sessions. We must score the most surf sessions in Vancouver, I am beating him this year as he skis as well. I used to have a few photo of ambleside SUP, amazing SUP surf photos, can't find them. With SUP you take the wave sooner and there is a longer ride as it does not sink when you lose speed.
Shortboards are an advantage when changing directions while in a wave, it's quicker and because it's more responsive it's fun. I guess it's like comparing light wind windsurfing vs high wind windsurfing.
I am almost 42 I use a shortboard, it's not the age, it's your muscle memory. You can swim free style at a swimming pool once a week for a year for upper body, work on your legs (soccer, etc). Once you feel more agile and balanced ... you'll enjoy more.
1-12 knots any SUP OK.
12-18 knots longboard OK.
18+ shortboard makes life easier, it sinks and there is no drifting while waiting for a wave.