The Portuguese tactician was speaking to Univision on his time as United boss.
The Red Devils are 12 points behind league leaders City, whom they lost to 3-1 before the international break.
Mourinho’s men host Crystal Palace this weekend as they seek to get closer to the top four places.
The 55-year-old admits there’s enormous talent at his disposal but the maturity level isn’t top notch.
“They are lacking maturity. I wasn’t a player, obviously, but when I say maturity I mean on a personal level.
“We had more men, we were more mature, we were more prepared for life, we were less protected.
“In English they call it ‘spoilt’, ‘spoilt kids’. Our kids are more spoilt than we were by our parents. We are more mature, we are better prepared. For young people now, it’s a different life on a social level compared to ours.
“The people who surround the players, who protect them too much, who give them – in my opinion – too much care, too many excuses. There’s always an excuse for everything.
Jose has had his issues with both Martial and Luke Shaw, keeping both players out of his team because they weren’t performing optimally.
Shaw however has been one of United’s best performers this season. While Martial has enjoyed a good last few weeks, scoring in five consecutive league games.
But Mourinho still believes both men alongside Lingard and Rashford needs to build their personalities.
“Luke Shaw, who is the left-back of the national team, who arrived there [at United] two years before me. He didn’t know how to compete. He had great potential, yes, but he didn’t know how to compete.
“We’re talking about Luke Shaw, Martial, Lingard, Rashford – young, big potential.
“But at the end of the day it results in the word I cannot say [cojones], but you use it a lot here… character, personality, they lack a little that natural aggression”