How can I find cooking methods that are more tailored to the age and state of vegetables?
The produce I get in weekly CSA box are all quite unique. I get at least 3 different kinds of kale, 4 different kinds of lettuce, and so on. Even with the same kind of lettuce, sometimes it's young and pale and other times it's dark green and hard to eat raw (but it's yummy when sautéed). The carrots in winter are sweeter than carrots in spring. Young onions are vastly different from normal onions.
The vegetables are all in high quality so I want to cook them with the most suited cooking approach (i.e., I don't want to simply substitue vegetables). How can I search recipes that are more tailored to the state of each vegetable? Or how can I teach myself to do that without looking up recipes?
By all means, teach yourself! Cookbooks and recipes are great - my collection is probably quite a bit larger than it needs to be - but in the end they are far more about learning than they are about defining how to cook. Your insight that each of the vegetables would benefit by being treated according to its individual qualities suggests that you will do quite well once you get your cooking "feet" under you.
Most cookbooks and most recipes do not differentiate between, say, different lettuces. But you can take a recipe for something similar and improvise. For example, you have already sauteed sturdy, dark green lettuce, just as one might saute escarole. Learn the techniques and apply them to the best advantage according to what you have. For example, sweet vegetables roast particularly well. Don't be afraid to fail, from time to time. When you fail, ask yourself what went wrong and what might have worked better. In the end, you will be a better cook for it!
I agree that you need to learn through experience, but want to point out that roasting is considered by many to be better used with older or non-sweet vegetables than sweet younger ones. The latter will be lovely whether steamed, grilled, sauteed, or eaten raw. But roasting is particularly good for the former as it caramelizes the natural sugars, making vegetables taste naturally sweeter. When caramelizing onions, it's best to start with yellow or other non-salad onions because sweet salad onions will come out TOO sweet.
When something tastes bland or too sweet, add a splash of vinegar to perk it up. Balsamic and rice vinegars are nice but others work too.
In general, I tend to do fresh young vegetables very simply, to bring out the flavour - steamed and lightly seasoned, quickly blanched, lightly sauteed or even raw.
Older vegetables are more suited to roasting, stewing, braising - longer cooking brings out the sweetness, and can mellow harsher flavours.
Things like pureed soups can go either way - but with different flavours.
For example - young carrots I might just grate, toss with lemon juice, olive oil and some toasted cumin. Older winter carrots might get roasted, or braised Italian style and topped with parmesan, or used in a carrot and ginger soup. Young beets with greens would be steamed and served whole with a bit of vinegar, mature beets would be roasted and used in a beet salad with lemon juice and Mediterranean spices.