Transfermarkt lists the average age of the United starting line-up for the last 10 PL games of LVG's reign as being: 25.8 (Bournemouth), 26.0 (West Ham), 27.4 (Norwich), 26.5 (Leicester), 25.7 (Palace), 26.3 (Villa), 24.9 (Spurs), 25.6 (Everton), 25.6 (Man City) 25.5 (West Brom)
For the first match of the season, long before Borthwick-Jackson, Rashford and TFM starting appearing in the team, the average age was 26.7. For the Villa game it was 25.9. Newcastle 25.6.
For the Cup final against Palace, our average age 26.9
Compare that to the average age of the United starting line-up listed on Transfermarkt for our recent matches:
27.6 (Brighton), 28.1 (Arsenal), 26.0 (Bournemouth), 28.5 (West Brom), 28.0 (Man City), 28.1 (Swansea), 27.3 (Liverpool)
FA Cup against Spurs 28.3
CL game against Sevilla home 27.7
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If you are more interested in the average age of the squad, rather than the starting 11, then selling McNair, sending Januzaj, Pereira and Borthwick-Jackson out on loan is going to have huge impact on the average age.
The mistake the OP made is ignoring the fact that all the players that Mourinho inherited, that are still at the club, are almost 2 years older now.