I'm also in my forties & I was just there in Sept 1wk on site solo, and then a second wk offsite with relatives. Had a 10day MYW with park hopper/waterparks... I went to Blizzard Beach 2x solo, Typhoon Lagoon 1x solo and once w/ the crowd.
It was great! And funnily enough, there were a lot of people that seemed solo, but many were actually just doing a slide that the rest of their crowd weren't doing/had done enough/were taking a break, etc.
People in the lines tended to have open conversations around, so that was pretty nice.
I absolutely had a great time at both parks, both solo and with people!
Preferred Blizzard Beach for the rides and for bobbing in tube in the wave pool. And the chairlift was great--single rider line!
But Typhoon Lagoon's Shark Reef was amazing.. there were people jumping out because it was too cold, but we found the water just normal temp--in Jul we usually go swimming in the Atlantic--icebergs always in sight.... that is very cold water then!
I don't know what temps are like in March, so that part I can't comment on.
But other than that: Go! Enjoy your trip!