Depends what ‘talking about sex’ consists of. Puerile kids banter or more specific, adult stuff. You mention something was of concern?
11 isn’t too young to be thinking about it. I’d say it’s fairly typical, even decades ago, pre-internet. But if he’s hearing about it from teenagers, you’ll have to talk openly with him to counter this. Nothing heavy, but drop it into conversations when the opportunity arises (while watching TV or talking about what’s in the news or related topics). Share your views about appropriate age, respect, consent, safe sex and porn.
I wouldn’t ban him having a ‘girlfriend’ (likely, quite literally), just as I wouldn’t ban him from having any friend, including the older ones who talk about sex. It could prove counter-productive, by making him more determined to see them or listen to them. Kids can always find a way.
But I’d keep an eye on it and not encourage it in any way. Be alert to preventing opportunities for them to be alone. Do you know her family? Does your DS go to her home or have the opportunity to be there when everyone’s out (e.g. on way home from school) or is allowed in her room?
It also depends on whether the older kids have older siblings. The youngest in a family tend to be exposed to stuff (sex, drugs, gaming, music, fashion), younger than the eldest, as siblings overhear, snoop on or copy each other. Or openly chat.