Its interesting on peoples interpretation of a "young manager"
Gerrard is seen as a young manager, he is 40 in a couple of months.
He is inexperienced a as a manager but I wouldnt say young, most folk are approaching mid way through their working life by 40
I think he is young in terms of football management. You don't get too many managers starting out under the age of 35 in the top leagues in Europe. I would suspect most managers begin management after having had a football career (thus 35+) and those who didn't, come into the role from other avenues - i.e. coaching - so still being in his 30's I think qualifies him as a young manager. It's relative to his field. Football jobs are not like normal jobs.
Looking at top jobs across England, Spain, Italy and Germany. I've split experience into new manager, middling and experienced based on my perception of the number of clubs they have managed/tenure managing first 11.
England:
Everton - Carlo Ancelotti - 60 - experienced
Tottenham Hotspur - Jose Mourinho - 56 - experienced
Liverpool - Jurgen Klopp - 52 - experienced
Manchester City - Pep Guardiola - 49 - experienced
Manchester United - Ole Gunanr Solskjaer - 46 - middling
Chelsea - Frank Lampard - 41 - new manager
Arsenal - Mikel Arteta - 37 - new manager
Spain:
Barcelona - Quique Setien - 61 - experienced
Sevilla - Julen Lopetegui - 53 - experienced
Atltetico Madrid - Diego Simeone - 49 - experienced
Real Madrid - Zinedine Zidane - 47 - middling
Valencia - Albert Celades - 44 - new manager
Italy:
Juventus - Maurizzo Sarri - 61 - experienced
AC Milan - Stefano Pioli - 54 - experienced
Internazionale - Antonia Conte - 50 - experienced
Roma - Paulo Fonseca - 46 - experienced
Lazio - Simone Inzaghi - 43 - middling
Napoli - Gennaro Gattuso - 42 - middling
Germany:
Borussia Dortmund - Lucien Favre - 62 - experienced
Bayern Munich - Hans-Dieter Flick - 54 - experienced
Schalke 04 - David Wagner - 48 - middling
Borussia Monchengladbach - Marco Rose - 43 - middling
RB Leipzig - Julian Nagelsmann - 32 - middling
Excluding Nagelsmann, who is an outlier and I would say unique example, managers of the top teams across the best 4 leagues range in age from late 30's to early 60's. Thus, at 39, Gerrard is a relatively young manager.
Also believe Gerrard is 3rd youngest manager in SPFL behind Jim Goodwin (38) and Stewart Kettlewell (35) with most in mid-late 40's and a couple are 56.