If a person go to the university at 20 instead of 18 or 19 is the same for companies ?
Not much difference. One graduate is off by just 2 years from the other graduates. If a company looks to hire someone for an entry-level position, the company would mostly be interested to know which candidates know how to solve the problems in which the company is interested.
One of the points of confusion in this thread is that the OP seems to be a lot more interested in the specific question of a student taking an extra year or two to finish a bachelor's degree (or master's degree) and the direct impact of that on the student's ability to find a job after graduating - NOT the more broad question implied in the title of whether ageism in general is a factor in hiring decisions.
People have different experiences. And when you ask a question like this on an internet forum, you're going to get an array of responses. But it's important to remember that the plural of 'anecdote' is not 'data.'
Unless someone can point to a study that says otherwise, I strongly suspect that for the specific question of interest - taking an extra year to graduate - there would be too high of a signal to noise ratio in any study to discern a statistically significant and practically meaningful difference. You might just as well be asking about people's ability to tell the difference between twenty-two year olds and twenty-four years olds based on appearances.
From a pragmatic point of view any difference in employability, if it even exists at all, is not likely to be so great as to necessitate influencing a decision to stay in school for another year or not, because such a decision has to be weighed against other factors such as the ability to build a network of professional connections, influence on grades, the ability to take extra courses and widen one's skill set, volunteer work, and even quality of student life.
Also, a year or two is in the noise. I don't know how the school system operates in Italy. But in the US, the school year typically (with some exceptions) starts in late Aug or early Sept. When a young child first enrolls in elementary school (e.g., kindergarten or first grade), there are certain age requirements that he must meet; these age requirements vary with locality. E.g., assume a child must start school if he has turned 6 by Sept 1, and that he is allowed to start school if he turns 6 by Nov 1 of the current school year. So, if a child turns 6 between Sept 2 and Nov 1, some parents will opt to enroll the child at the start of the current school year if they believe the child is ready. Other parents, however, do not want their child to be among the youngest in the class, and feel their child will do better if he is among the oldest in the class; these parents will opt to enroll their child the following year. So at the start of any given school year, there will be children turning 6, along with children turning 7 ... about a one year spread.
Additionally, in recent years, some seniors graduating from high school are opting to take a "gap" year before starting college. I don't know the statistics on how many are doing this. But over the past few months, I've talked to four or so who are. So if you take into account the different age in starting elementary school, along with a voluntary gap year, you have a two year spread in starting college.
as mentioned above, it is extremely common in the US for indergraduates in college to take more than 4 years after their graduation from high school to finish college. In the 1960's, I recall that perhaps 25% of each class at Harvard took a year or more off before graduating. I did so myself, and my three roommates took 1, 1 and 2 years off. This had essentially no effect on their future careers. Moreover it is common for students to spend an extra year after high school in preparing for college, especially athletes and others desiring to gain entrance to an elite school and hoping to improve the chance of acceptance by taking an extra year of preparation at a "Prep" school. This was suggested as an option to my son who hoped to gain acceptance to an elite college and play basketball there. It is an advantage for a college athete to be older since a 25 year old makes a more mature athlete than an 18 year old. Indeed one classmate of mine, Christian O'Hiri, regarded as the greatest soccer player ever to play at Harvard, entered as a freshman at the age of 22, after already playing on an Olympic soccer team. It was seldom even mentioned, as he dominated ivy league soccer, that he was 4 years older than the competing players. I myself graduated college the year I turned 23, and my two sons turned 23 and 24 in their graduation years. None of these age differences affected any of us to my knowledge. I also delayed obtaining my PhD, receiving it at the age of 35, rather than say 25 or 30. This played no role in my competition for a place in graduate school or for a tenure track job. It did however play a role in determining the number of years during which I was able to accumulate retirement credit after being hired. I.e. people usually retire in the US between 65 and 70, so a person who starts work at 25 or 30 thus has more credit toward retirement than one who starts at 35 or later. (Technically professors with tenure do not have to retire at a given age, but univertsities can do things to encourage them to do so, such as make the retirement salary calculation go down for older workers, or increase the burden of their teaching load.) But basically I think we are saying you do not have to worry about how starting uni say at 20 will affect your future. I.e. for a young person, a time variation of even 4 or 5 years probably makes very little difference.
This is a very good answer to my question.
@mathwonk thanks a lot for sharing your experience in this thread !
In my experience, it depends on the quality standards of the company. I've worked for companies that exclusively hired young, fresh-out-of-college types simply because it was way cheaper. After a decade in engineering, your starting salary for a new job doubles, so start-ups tend to stick to the lower end. Big companies or established companies will pay more for extra experience.
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