How we cite our quotes: (Book.Chapter.Paragraph)
Quote #1
The amiable youth [Bilham] then look out, as it has first struck Strether, at a world in respect to which he hadn't a prejudice. (3.2.45)
The thing that Strether probably envies the most about Bilham is that the guy is just too young to be set in his ways. Strether feels that rigid thinking is probably the number one cause of unhappiness in his life because it has led him to steer clear of exciting new experiences. Instead, he has let the years get the better of him and has settled into a quiet, boring life. He never would have even realized this, though, if he hadn't come out to Paris to meet Chad and his young friends.
Quote #2
If he was himself moreover to be treated as young he wouldn't at all events be so treated before he should have struck out at least once. His arms might be pinioned afterward, but it would have been left on record that he was fifty. (3.2.84)
When he first runs into Chad, Strether feels that he wants his age to be acknowledged by the young man—in other words, to be respected as an elder. This is the exact opposite of what Strether will want later in the book, but for, this definitely a Woollett way of thinking. And as the book unfolds, Strether will realize that his age doesn't really entitle him to anything. It's his quality as a person that'll make people respect him.
Quote #3
Chad accordingly, who was wonderful with both of them, was a kind of link for hopeless fancy, an implication of possibilities—oh if everything had been different! (5.1.5)
Seeing Chad live such a cool Paris life makes Strether feel a twinge of regret about wasting his own youth. To be fair, the dude lost his wife and his son ten years apart and spent a lot of his life mourning. But even before that, Strether feels like he has always been too safe with his life and has always given into fear. He badly wishes that he were still a young man strutting around parties like Chad and talking to women without feeling ugly and old.