There’s a slight difference between ‘being loyal to the job” versus ‘being loyal to a person”. Kattappa has always been loyal to his job no matter who the king/queen is. But hasn’t been loyal to individuals.

Kattappa is a loyal servant, an obedient slave. In other words, he had a slave mindset. He never gave importance to his personal opinion, or what was right or wrong. He strongly believed delivering justice is the job of the king/queen and his job is only to follow orders, no matter how absurd it is.

• When Rajamatha Sivakamidevi orders Kattappa to kill traitors within the kingdom – he does it. Not because he’s a loyal servant to Rajamatha but because he’s a loyal slave to the queen of Mahismathi.
• When Amarendra Bahubali quit the kingdom following the orders of Rajamatha and went out to lead a normal life, Kattappa stayed back with Rajamatha and his new king Bhallaladeva. Here his loyalty to the kingdom supersedes his loyalty toward Bahubali
• When Bhallaladeva orders Kattappa to kill Amerandra Bahubali – he does it. Not just because he got orders from the king but because he believed Rajamatha also wanted Bahubali dead. Here his loyalty is towards his job of being an obedient slave to the kingdom than a loyal uncle to Bahubali.
• However, when Bhallaladeva tries to kill Bahubali’s son Mahendra and Rajamatha, Kattappa steps in to save both the kid and the queen from Bhallaladeva. Not because he’s dishonest to the king but because his loyalty has never been to the king but to the superior of the kingdom who’s Rajamatha. In other words, his loyalty is towards the kingdom and not the king.
• When Devasena escapes with Mahendra Bahubali, Ballaladeva orders Kattappa to bring her back. Kattappa concentrates only to bring her back without concentrating to protect Ballaladeva’s son. Here again, he follows orders. His job is to bring her back, not to protect the future king of the kingdom. To bring her back, Mahendra is an obstacle, so he fights with him.
• When Kattappa realizes that Mahendra is still alive, he switches sides. Not because Mahendra is the son of Amerendra but because he was the “declared king” of Mahismathi by Rajamatha before she died.
• Till the end, Kattappa concentrates only on following orders as he’s loyal to his job of being a loyal slave.

Answering the original question – How old is Kattappa when he becomes loyal to Amarendra Baahubali?

Despite Amarendra Bahubali calling Kattappa his “uncle”, Kattappa has never been loyal to Bahubali. He had always been loyal to Rajamatha who’s the superior of the kingdom. He does whatever she asks him to do, including killing (or backstabbing) Bahubali.

This is because Kattappa’s loyalty is only to his job, his loyalty is only to the kingdom – not to any individuals in particular.

“How old is kattappa” — how to define age when loyalty to Bahubali never existed in the 1st place.