Poultry Talk Ontario (pto.ca)

My 5 light sussex hens are now 15 months old. They were laying well over the winter months....3-4 eggs every day from the 5 birds. In the past couple months I'm getting only 1-2 eggs a day from them, some days 0. Are they just going thru a slow period and will pick up again? Or are they now well past their prime?

I doubt that they are past their prime at 15 months: My speckled Sussex is about 2.5 yrs old this year and is still laying 4-5 eggs a week.
All my hens slow down a bit during the summer months when the weather is really warm/hot. I think that's normal.
I would check for lice or mites, though - that can sometimes impact the number of eggs laid. Have they had any new stressors in their environment lately? Something different in their housing or routine? Change of feed? Or is it possible that they are going into their annual moult already?

My 15 month old pair are averaging 8 or 9 a week. The older ones less. I'm considering selecting for laying ability in the 2nd year and beyond. They shine in the meat department.

They might be slowly heading towards molt, August to October is the most common time.
They will start laying again with additional light to give them 12-14 day light, they will lay thru the winter, without light some will lay and some wont's and all will start again as the days start getting longer in January -February
Sussex are dual purpose, so the egg production isn't as good as breeds bread for eggs only, but for dual purpose they are very respectable layers, and lovely to look at to boot. :)

I wondered about molting too. Although I haven't noticed missing feathers. There's one or 2 that were acting broody a few weeks. They all look the same so I don't know if it's always the same hen on nest or not. No changes in environment or feed.

It really sounds like slow start to molt, I find the Wyandottes have pretty much the same pattern of laying, the first year pullets hatched in May June, will start laying in September October and lay until next August, September , so if you have hatched a new batch of chicks, they will just start laying when the original group starts slowing down, preparing for winter.
If you add a new group of pullets every year, it guarantees pretty much continuos supply of eggs and as long as they all look healthy, most people don't worry too much who laid what, as long as "somebody" is laying.
Broodiness is also a possibility, there seems to have been an epidemic of that this year.

These were hatched in early april last year so molting does sound likely. Will they lose their feathers? How long do they not lay?

Some you will hardly notice molting, some will pretty much denude themselves.
Once they start it takes about 3-4 month, you can watch the progress , molt starts at the head, then neck, back, breast, wings and the tail is last.
Growing new feathers requires lots of protein, thats why the hens can't lay eggs and molt at the same time at least most don't.
If you think of it in terms of birds in nature makes sense, all the young have been raised, winter is coming , good coat is needed for the winter and then save up energy for next year to start laying eggs to hatch next years chicks.

Thanks for all that! I will need to watch closely to see how they progress.

Make sure they're drinking enough too because eggs need a lot of water and can dehydrate the hens. You can add vitamin and electrolyte supplement to their water if you wish. I do this at times of stress anyway such as moult. This is not the same product as superbooster (which contains an antibiotic).