For this one we kind of need to read the previous lines to understand it:
Vidi ego, qui iuvenum miseros lusisset amores,
Post Veneris vinclis subdere colla senem
Et sibi blanditias tremula conponere voce
Et manibus canas fingere velle comas,
Stare nec ante fores puduit caraeve puellae
Ancillam medio detinuisse foro.
I have seen the man that had mocked the hapless loves of the young, in later time put his aged neck in the halter of Venus
and make soft speeches for himself in quavering tones and turn his hands to dressing his hoary hair :
nor did he blush to stand before the doors of the beloved or to stop her maid in the middle of the forum.
which the Loeb edition translates as
And then the hard part:
Hunc puer, hunc iuvenis turba circumterit arta,
Despuit in molles et sibi quisque sinus.
I don't get why 'puer' is singular though clearly since 'circumterit' is singular it is not a mistake. I also don't get what case or part of speech 'iuvenis' is. It seems that it can be both noun or adjective, and although it is always singular, it seems to be a collective noun that is singular grammatically but plural in meaning, such as 'sand' in English. By the way, the Loeb translation is:
Round him boys and young men pressed in a jostling crowd, and spat each into his own soft bosom.
At first I thought the first 'hunc' might be the object of 'circumterit' and the second 'hunc' the object of 'despuit' but what about 'sibi'? Couldn't 'sibi' be the object of 'despuit'? Although there are not too many examples of 'despuit' in the OLD, none of them take a dative. Still, if 'iuvenis' is a nominative adjective that modifies 'puer' then it seems that it would mean
A young boy pressed around him in a (small? compact? dense?) crowd
But my theory goes awry when it seems that 'quisque' means 'each member of the crow' which doesn't make sense because why would you talk about one member of the crowd crowding around the man, then go on to talk about the actions of the crowd itself. In any case,
and each one spat in his soft chest.
Still, I can't make sense of the second 'hunc'.
We also might as well talk about the meaning of 'arta'. I don't think 'jostling' is a good translation. I think OLD 7 is what is meant here: "Small or restricted in size, confined; containing few members, small in number; (of a number) small"