dumb question but how do I know what's old and what's new?

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usually thickness.. and bark maturation ...

look at this years growth ... probably still green ... trace back.. last years is brown ... the year prior might be starting to show bark ... etc ...

just take the time to figure it out ... bug spray and a 6 pack.. youll figure it out.. wine if you prefer ... lol ...

next.. get in there.. down near ground level .. you will see .. one inch stuff.. two inch.. 3 inch.. etc ... you will see a pattern .. and usually .. with renovation pruning ... you just decide to take out the largest of the bunch ...

i have never pruned a ninebark ,... so i hesitate to go further with suggestions ... im just explaining.. how to look at a tree/shrub ..

so the next day.. or weekend.. after you sober up.... get out the sharp tools... if you insist ... in the video.. he waits until fall ...

frankly... the ones in that pic are gorgeous.. and fit perfectly in that bed.. i dont know why you want to do anything ... so i wonder what your goal is?? ... most of us.. dont prune.. just for the heck of it.. or because we have to .. we usually have some precise goal.. which helps us define where to go with it all ... and once you define a goal ... then actually doing it becomes easier ....

looking at other plants.. might help you define you goal ... https://duckduckgo.com/?q=summer+wine+ninebark&t=ffcm&iax=images&ia=images

dude.. move the metal tag first ... note the older bark.. on those he cuts out.. they are all the oldest by looking at such: https://www.youtube.com/watch?v=uHr7gWVDO9o

it really helps to do it leafless.. when you can see all the structure ...

and finally.. peruse these ... maybe you will find your muse: https://duckduckgo.com/?q=summer+wine+ninebark&t=ffcm&iax=videos&ia=videos&iai=uHr7gWVDO9o

not mant healthy shrubs or trees.. die just from pruning.. and even if you screw it up royally ... they will regenerate.. and you can try again in a few years ... so dont worry too much about it all ..

ken

Whatever you do, wait until late winter or very early spring.

They look fine to me, if anything still on the young and floppy side. So I think you can soon end up making them too thin, even less vertical than they already are if you do much cutting at this stage - the cutting out oldest canes thing is something you start doing when you see obviously over the hill ones among the others, ones that aren't as productive as they should be.