I'd have to guess inadequate fertilization, though you say you have others that are doing fine. If they germinate, the seeds are fine. Seeds lose viability with age, but viability is a matter of germination-ability. It is true that older seeds have less energy stored for germination, but once they germinate, they're not getting much from the seed anymore. That is puzzling.

I'm chalking up a lot this year to the weather, the nights have been unusually cool where I am, even early this morning, in what should be the dog days of summer, I had on a long-sleeved shirt because it was chilly. I think that really affects warm-season vegetables. Though, maybe some aren't as sensitive, IDK. My pattypan squash and kabocha squash are growing like gangbusters, while the butternut, which usually is unstoppable in growth and fruit set, are just kinda sitting there with only a few measly squash out of 6 vines. {shrug}.

Thanks, Dan. This is my thought, too - need more nutrients. But, I'm gardening the same as always - paying attention to individual plant needs - fertilizing, water, amending soil, etc. A honeynut squash vine is going gangbusters but the yellow squash next to it is the same as it was back in May. Basil and dill growing well but pattypan squash next to them hasn't grown in weeks. This is so odd and new to me after 50 years of successful veg gardening. I normally have a very green thumb. I guess next year if new seeds grow normally, I'll assume it was some sort of weird seed thing. I keep replanting to see if anything changes, and I've had nearly 100% germination, so now I have a ton of tiny sprouts - if they all decide to grow into mature plants, I'll be in trouble with too many plants for the space. At least, it's nice greenery to have in the yard. I don't have a dedicated veg bed. Most veg are interspersed with flowers and shurbs - but it's all grown beautifully for decades that way.

One plus - with so many squashes not leafing out much past a couple/few sets of true leaves, there's definitely been less SVB and squash bugs to deal with. I guess I should be grateful for that.

BTW, other 5+ year old seeds are growing fine - it's just the older squash seeds that have the stunted plants.

That's especially annoying to have seedlings that don't grow. With regard to old seeds, you just plant more, and tolerate a lower germination rate. But once they come up, you just assume that you're good to go. To the extent that these are interspersed with flowers and shrubs, are they getting decent sunlight? Those flowers and shrubs may also be competing for soil nutrients.

Dan - all the vegs are in full sun; some plants are next to shurbs but not "under." And I feel like I'm feeding them enough - at least similar to how I've done in the past with solid results. Generally, I don't plant the same vegs in the same spots - I tuck them into open spaces amongst the garden, which changes as the perennials reseed, or die from old age and leave a hole, etc.

I guess tomatoes will be my most plentiful harvest this year. I only put in two seedlings, but I let the volunteers do their thing. Most years I yank them but every now and then I'll let one or two grow. No idea how they'll turn out but happy to see them grow. One is growing in the crack between the driveway and the stepping stones next to it. I expected it to die but it's about 3' tall now, resilient little bugger since I've never watered or fertilized or anything, it's not even staked.

Maybe Mx is right - chalk it up to weather. We had a drought and weird spring/early summer coolness, then too hot-too fast. The happiest plant seems to be Driveway Mystery Tomato. :)