Bubble chart in d3.js with some threshold circle

I’ve created a bubble chart of a simple biostat data with varying sized circles based on a person’s age.

What I’d like to do is create a threshold circle so that a node with age greater than some threshold would be inside the circle and rest will be outside. Larger the circle(the one with higher age), the closer it should be to the center. I’ve used d3-force to do this but it isn’t consistent.

My approach :

For some threshold t, the force on the nodes with age >= t would be towards the center of the circle. For the rest of the nodes, the force would be towards the circle’s parameter. But this isn’t consistent, sometimes it just looks like a mess : https://i.imgur.com/izUK4ta.png

Is there a simpler/better way to do it?

Here’s my current code
bubble chart (github.com)

Any kind of help would be appreciated.