Old age should burn and rave at close of day; (line 2)
It's pretty obvious from this line that the speaker thinks the elderly should fight death with all their remaining strength. But there's also a subtle implication here – they usually don't. That's why old age "should burn and rave," instead of "does burn and rave." Usually, the speaker knows, the aged slip away peacefully. This really bothers him; he doesn't want there to be anything peaceful about it.
Grave men, near death, who see with blinding sight Blind eyes could blaze like meteors and be gay, (lines 13-14)
Being close to death seems to make it possible for old men to have a special revelation – that, even in death, they can live with a fiery intensity and find a strange, ecstatic happiness.