As long as no kids are using the playground equipment at the park it's perfectly okay for you to play on the playground equipment at the park as long as you can fit on it.
It is okay for an adult or anyone of any age to play on a swing at the park.
Although if there's other kids in the park it's best to avoid the swings and playground equipment unless you have kids with you.
Let the kids play on the equipment but if no kids are around and they are not playing on the swings etc then it's okay to get on the swing and play.
I sometimes go to the park in the evenings after work or early mornings before work and play on the swings for a bit and even go down the slide.
I'm in my 30s and will be 40 pretty soon and I still enjoy playing on slides, swings and even climbing trees.
I also have a swing set and slide at my house for my kids and sometimes I will also get on it at home as well with or without the kids.
The park though is near my job and before going to work if the weather is nice enough I will go to the park and play for a bit.
Or after work I will go to the park and play for a bit before coming home as it helps me unwind from a day of hard work.
You're never too old to play and we all pay taxes for that playground equipment and park so we also have a right to play on the swings and slides and other playground equipment we can play on.