I don't think age is that important because I won cup and CL with team which had age average 20 (one player was 30y.o., one 23, all other 22 or less in 22 players squad)
What is important then? Form.
Form is something that i think it is somehow written in your player's genes. If player has constantly bad form, you can't change it whatever you do. I've tried with blue boosters, position changing (for MC's), contract renewal (to increase wage), increase win bonus... Nothing worked. Only possible solution is to sell such player and get new one. It doesn't have to be whole team. Change only worst form players, few of them can change outcome drastically.
I've went through that, my team had 9th position for days although had strongest squad in the league. Then I sold my MC's, their form was constantly 6-7. Form of team completly changed, at the end I managed to finish season 2 points behind leader.
It is hard to avoid troll results, but if your team is in bad form, they will repeat and repeat. One troll result doesn't mean your team has problem.