Looking at Andre Onana's custodian career, you might notice that he's not just starting out at 27. No young bird, he is. Quite the contrary. Before joining Manchester United, Onana was a security guard for Ajax and Inter Milan. Important and serious. If Onana's performance seems choppy or unfocused, he may be still getting adjusted to Man United's style. His performance measures may have changed due to the complex and crucial adaption process
Goalkeeper don't have to be young, most of world star goalkeepers are mature, only few exception e.g. De Gea, Neuer etc.
And what is security guard? Onana is a goalkeeper who play in professional football scene, he's not someone who work as a security in Ajax and Inter.
Goalkeeper is an independent role, there's no situation a goalkeeper can't fit in someone club, it mean he's a bad goalkeeper and he need to train harder.