There is an onerous confirmation process to get nominated candidates approved as judges on SCOTUS. People who have not demonstrated sufficient judicial knowledge and aptitude are supposed to be turned away or discouraged off.
This for example happened to Harriet Miers, one of Bush's SCOTUS nominees, who withdrew.
Now, some considered Barrett a bit lightweight in that domain. But she wasn't as extreme a case as posited in this question.
The City Bar finds Judge Barrett to be “an extremely talented lawyer and judicial writer” who “unquestionably” meets the first three of the City Bar’s evaluation criteria: (1) exceptional legal ability; (2) extensive experience and knowledge of the law; and (3) outstanding intellectual and analytical talents.
Nominating a brand new judge with limited experience, solely for the transparently partisan purpose of ensuring a long tenure ought to be too much to stomach for at least some individual congress persons, thus endangering the nomination. Ought to, anyway.
Basically, how comparable is a 30 yr old lawyer nowadays in terms of experience and expected lifetime to live to a 30 yr old in the early 1800s?
As far as historical ages of nominees, most/all(?) of the linked examples were the 1800s, a time period where the average life expectancy in the US was around the mid 40s (in 1850). So their relevance to comparing to nomination ages nowadays is open to interpretation.
See the comments below for criticism:
"That's because of high levels of child mortality. Once somebody survived to adulthood, the life expectancy wasn't that much shorter than that of today."
but as someone else stated: The source says it used life expectancy at age 20, so child mortality shouldn't be a factor here. I suspect it's showing life expectancy as "expected remaining years" rather than "expected age at death" the data is a bit more open-ended than that, is it states that in some areas at least, once someone reached the age of 20, they had about 40 more years to live. e20 in table 1. That's a life expectancy of about 60 years so not that great.
- life expectancy in the 19th century was higher in the countryside than in the (unhealthy) cities. SCOTUS judges lived in the (quite unhealthy climate) Washington.
Lawyers took the bar at an earlier age (3 out of first 4 I looked at that were young in the list took it at 21-22. One at 27). Additionally, lawyers apprenticed early on, so could conceivably have more practical experience than today's grads.
In the 18th and 19th centuries, most young people became lawyers by apprenticing in the office of an established lawyer, where they would engage in clerical duties such as drawing up routine contracts and wills, while studying standard treatises; this became known as reading law. The apprentice would then have to be admitted to the local court in order to practice law. Frank B. Kellogg (1856-1937) is an unusually successful example of this route. Starting as a farm boy in Minnesota who dropped out of the local one-room school at age 14, he never attended high school, college, or law school. He clerked for a lawyer who specialized in corporate law, and soon proved himself adept. He played a major role as special assistant to the U.S. Attorney General in one of the most famous decisions in corporate legal history,
Basically, before the 20th century, people often assumed positions of leadership at an age that would seem precocious nowadays.
I am leaving this up because I think it merits consideration. But, at the same time, the remarks about the potentially misleading impact of childhood mortality on the overall stats bears keeping in mind.