Actually, Roberto Firmino is still 32 years old and this is not too old because if he can continue playing well, he still has at least 2 to 3 years before he really experiences setbacks, but it is not clear whether Roberto Firmino will be accepted if he wants to return can enter the Premier League.
There, talented young players are far superior and more in demand, if the competition is tighter then Roberto Firmino chances of returning are only slim, but if that fails, he will inevitably have to survive and get used to Matthias Jaissle leadership style.
I think it will be very difficult for Firmino to return to a big club in the Premier League. Because as you said, competition in the Premier League is currently very tight. In fact, it is not only old players who can be eliminated there, young players are often eliminated if their performance is not good. So looking at these conditions, it can be said that Firmino will most likely have difficulty returning to a Premier League club. Moreover, currently, in my opinion, Firmino's performance has decreased slightly. Because it was proven that he was no longer called up to the Brazilian national team for the Copa America competition yesterday. Even at Al Ahli, his performance this season could be said to be mediocre. So with this fact, it seems like Firmino will have to enjoy the rest of his career with Al Ahli. The reason is, the hope of being able to return to playing in the English League or the big European leagues seems very unlikely.
So it is not impossible, for example, that after Firmino's contract ends with Al Ahli, he will choose to retire. Because after his contract ends with Al Ahli, he will definitely get older and most likely his stamina or skills will decline. And if it's like that, it's likely that no club will want to recruit him (let alone clubs from the big European leagues).
Apart from that, next season Al Ahli will loan Allan Saint Maximin to Fenerbahce. If this is true, then Al Ahli will certainly experience a decline in performance in the left wing sector.