It appears to be somewhat weathered, but overall solid.
You can do as much or as little as you like to it, from there. It's a deck, not a living room floor. If it's not structurally compromised (and it does not appear to be) such that falling through it is likely, it's doing its job. If some boards are, replace them. If more than half the boards are, consider it might be time to replace them all, at that point.
Beyond that is cosmetics, with a touch of perhaps preserve its remaining life a bit. Sand if the splinters are bad enough to annoy you, or don't if they are not; then seal it, oil it, stain it, or paint it as you like, if you want to. Hopefully it's pressure treated material so rot should be limited.
Sanding back so it looks like "new wood" before you seal, oil, stain or paint is purely cosmetic and not needed for any reason other than trying to make a deck out in the weather more of a project than it needs to be. If it's stabbing you with splinters, that's a functional reason to sand, but making the wood "not weathered gray" is purely cosmetics. That's what happens to wood in the weather, and plenty of structures built like that function for decades with no further cosmetic treatments done to them.