Sorry I didn't quite write that properly.
What I meant is....
If she HASN'T got those acute symptoms I mentioned, then she can probably wait till the vet's office opens.
If she HAS got any of them, then take her to the ER vet.
No, it's wise never to consider a sudden change in energy, general health or fitness as just "getting old"
Usually those changes mean someting isn't quite right, and needs attention.
Whether she needs emergency care...I'm not sure. That would be up to you. I would say that if she hasn't eaten or drunk anything the last 24 hours, hasn't passed water or pooped in the last few hours, or is unable to get up from her bed, vomiting, diarrhea, or showing signs of pain (shivering, whimpering, restlessness, etc) or any other acute symptoms, then you could probably wait till normal vet appointment hours.
But it might be wise to take her in today or tomorrow to your vet and get her checked out.
Hopefully it won't be anything too serious. Even though she may be getting on a bit in years, it doesn't HAVE to be serious.
But go get her checked out. It can't do any harm.