Yes, it's definitely a deliberate choice to pair up Hazel with a much older Agnes. Hazel would be around 30s to 40s, while Agnes is at late 50s to 60s.
The writers could have chosen a younger actress to play as Agnes, make her more innocent and optimistic. But no, we have a 60-years-old Agnes smooching with Hazel, and it ain’t disgusting at all.
(Edit: That is not to say that I intend to see 30-year-old Vanya and FIve in a thirteen-year-old body smooching up, read on.)
Despite the huge gap, you don’t see the balance tipped over to Agnes for being old, nor is it tipped to Hazel because he is not ordinary like her.
The age gap has to be glaring in order to frame a similar pairing. I wonder what that pairing is?
(note that I’ve edited the last sentence from “foreshadow” to “mirror” to “frame.” I realize that frame is the actual word I’m looking for. Foreshadowing is often used as a warning to predict future events, though it can be used to mean something positive. It’s just that it is shown to us audience and not to the character themselves, like no character has to point that out. Mirror felt like it already has drawn a lot of parallels to it when fiveya has yet to initiate an official relationship. Frame indicates that it’s more of a potential, and a possibility instead of it being final considering that the showrunners could scratch it if they wish.)