Age is not a factor for a spinner. It doesn't matter even if he was younger. If you live of past laurels, don't work hard on your game and don't develop new tricks, don't learn to disguise your existing tricks, don't strategize and plan on how to get around batsmen developing counter plans against you, you will struggle to survive for long regardless of whether you are 20 years old, 30 years old or 36 plus. This game is all about adapting, making adjustments, developing your skill sets and strategies if you want to have a long and effective career.
Murali is a classic example, he relied heavily on his huge turning off spinner from 1995 to 1998. Kallis showed the entire world how to counter him in the 1998 CT by continuously slog sweeping him. Murali adapted by adding a doosra (which he made sure was well disguised and not easy to read/pick unlike Saqlain) from 1999 onwards and he became a bigger force in world cricket. As he got older and had more wear and tear on his body and shoulders his ability to generate massive spin on the ball started to diminish and the English batsmen in the 2002 series in England showed the entire world how to counter Murali by sweeping him but making sure that their front pads were always outside the line off the off stump therefore taking the LBW out of the equation, this tactic didn't stop Murali from picking 5-6 wickets per innings but the biggest difference is that instead of picking 5-6 wickets for 60-70 runs in 30 overs, he was now having to bowl 50-60 overs for his 5-6 wickets for 130-140 runs and that makes a huge difference. Even the Australians the invincible side of the time commented that they took huge inspiration from England's tactics against Murali and would apply it.
But this didn't stop Murali, he learnt to be more inventive with his field placings and started to go around the wicket to the right hander and spinning the ball into the right hander to make up for his loss of massive spin, revolutions on the ball so that batsmen employing the sweep against him would still get hit in front of the wickets or would be caught bat pad and he employed this tactic and approach from 2002 to the end of his career.
This is what world class bowlers do, they adapt to the opposition and the tactics employed against them. This is why Murali is a legend and Saqlain is regarded as someone who got exposed and useless when batsmen got used to him because he failed to adapt. To be fair to Saqlain, he is not the only one, Ajanta Mendis is another example.