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I have a habit of sticking citrus seeds in the soil of a handy overwintering plant, then separating them out when and if they grow. For a while I didn't bother to label the seeds so I have a bunch of seed grown little trees that are unidentified.

Last year, I poured over online citrus identification pages and tentatively ID'd this one as a lemon or a Mandarin orange. Today, I was wondering if the bronze new leaves have any significance, and it seems that lemons are more likely to have bronze new foliage.

... Ignore the light colored foliage in the background -- that's another citrus that didn't take kindly to being left outside as long as it was. (I'm assuming its a hint that that one is a more cold sensitive citrus compared to the "lemon")

Nice.
I didn't know citrus trees grew in NJ. It is outdoors? We have one that grows similar leaves (a lemon tree),
but we are in So. Ca.

I have to bring them all in before it gets too cold. I hear it's possible to keep them out until they do go dormant or nearly so, then keep them in a sunny unheated but protected from super freezing temps location, but what I do is bring them in before they get too cold -- few nights in the 40's upper 30's and daytime temp in the 50's, then bring them inside for the duration of the winter. Initially, they are moved from an open sunny location to a protected part sun location as fall arrives.

Once inside, they can make it with just a sunny window as foliage house plant and they are fairly easy keepers with good drought tolerance, but I believe they need supplemental lights for blooming and fruiting unless you have a really good location like south facing sliding doors/picture windows or a sunroom.

I put them outside around same time as when I plant tomatoes and they spend the summer on outdoor vacation, starting in a protected location then gradually moving out to full or nearly full sun area.

Now that we're past last average frost, I moved almost all of the citruses outside. Only ones left inside are the 4 small kaffir limes from cuttings just because I seem to remember lime is more sensitive to the cold... Even though I did migrate the suspected Tahitian Lime outside... And the afourer mandarin seed-grown group in the big planter, which needs two people to move.

These are most of them (two containers were moved out a couple of days ago to the other side of the house) ...the front two in small terra cotta clay pots with funky trunk-base structures are my Bonsai Wannabe citruses.

I have a grapefruit seedling as a result of one of my young daughter's "science experiments." It has finally started to make a second set of leaves. I've been putting it outside in the day and indoors at night. Do you have any basic advice?

Citrus from seed can take anywhere from 3 years to never. However, if you take cuttings from a fruit bearing tree, the cuttings will usually bear in one to two years once the plant is big enough to support fruit. It will be a clone of the original.