Naming has a large arbitrary and cultural component and so is subject to the same structures as fashion. It seems for many centuries the only choices for Western Europe were Christian saint's names and for men mostly just the apostles.
But fashions change. It seems like in 19th c. Americans, the fashionable source was Old Testament prophets with z's and k's (Ezekiel, Zebulon, Zachariah).
And lately (early 21st c) American girls are named after president's last names (the aforementioned Madison, plus Kennedy, Reagan, Taylor (this is the answer to one of the questions). This is simply a current fashion trend mostly likely to be replaced soon from some new set.
One subtle pattern in English speaking areas for at least the past two centuries is to name a boy using the maiden name of the mother, usually a second or third boy since the first got the full name of the father (with Jr. or III appended).
As trends go, people sometimes follow the external view of the phenomenon (last names for first), which accounts for Jefferson Davis or (much later) Harrison Ford, whose mother's maiden names did not supply their first names. That these first names are the last names of famous people (American presidents) probably helped in their choice.
Best Answer
William Pitt the Younger was Prime Minister of Great Britain from 1783 to 1801 and was only 24 when he took office but that is not why he was called 'Younger'. He was called 'Younger' because he was the son of William Pitt the Elder, who was the informal leader of the British Cabinet from 1756 to 1761, so the adoption of the titles is not confined to Roman times.
William Pitt the Younger