You've asked two separate questions here. I'll try to approach them in order:
Why Saito is older while Cobb remains the same age
You're absolutely right. Saito is older than Cobb because he's been down in Limbo much much longer. Not only is there a dramatic time differential but he's also unaware of the dream nature of his existence and is expecting his age to continue increasing.
ARIADNE : How long would we be stuck there?
YUSUF : You couldn't even think about trying to escape until the sedation eases
EAMES : How long?
YUSUF : Decades - it could be infinite - I don't know! Ask him - he's the one who's been there before!
Cobb later clarifies:
ARIADNE : And if he dies?
COBB : His conscious mind will drop out of the dream. He'll be trapped in Limbo for a lifetime...
Washing up on the beach (again)
This one's a bit trickier. Cobb shepherds Ariadne out of Limbo, then in the next scene we see him back at the beach.
Ignoring any out-of-universe answers, the simplest explanations are:
Cobb briefly left Limbo when Mal died (possibly but highly unlikely)
Cobb was so distracted when Mal died that it somehow resulted in him returning to Limbo location #1, the Beach (possible but unlikely)
Cobb has spent years searching Limbo for Saito's fortress. The beach just happens to be where we catch up with him (this imho is the most likely option, especially when we take into account that Cobb has aged and grown a beard, indicating that a great deal of time has passed.
Out of universe, it's a nod to the idea that dreams often take non-linear paths.
Nolan speaks to the fact that any scene involving Cobb is filled with ambiguity and misdirection;
"He’s an untrustworthy narrator"
"...you need to doubt Cobb"