Avocados often do poorly during the Winter, but that's no reason to abandon them.
I've had pretty good success with mine, and I'll link to the Thread I started on this subject.

I provide my plant with a well-draining soil that can be watered thoroughly, and a sunny window.
Can you re-locate your plant near a window?

Yellow leaves often result from over-watering - or a soil that holds excess moisture.
In *most* plants, browned leaf-tips indicate dead roots/root-tips (As Above, So Below).
However, with Avocados, I think browned leaf-tips are par for the course (indoors),
in light of the low humidity and lack of adequate air circulation in most homes.

Before I can advise you further, I need to know what kind of soil you're using,
what kind of container and drainage is provided, and what sort of fertilizer you used.


Josh

Also, I'm glad that the plants have plenty of light.
That's good to hear, as it allows us to eliminate one more variable.

How long does it take for the potting soil to dry out (deep in the container)?

I assume that the problem lies with the potting soil, which sounds water retentive.
If the primary ingredient is peat moss, it's almost assuredly staying too moist.
As you know, this can interfere with nutrient delivery and uptake. This is why I hesitate
to recommend fertilization...it could be that there are nutrients in the soil, but
nutrients that are simply unavailable to the plant.

I will tell you that I fertilize my plant with Foliage Pro 9-3-6, which has higher Nitrogen
in relation to the other major nutrients. It also includes all the minor nutrients.
When I fertilize, I also add white vinegar (a capful) to the watering solution -
but I can only recommend this for the potting mix that I'm using. I don't know how it will
affect a soil with different drainage characteristics.

My sister grew a couple Avocados in peat-based soils and the plants declined eventually.
I actually decided to grow an Avocado to see if I could find a way to keep these plants
happy both indoor and out, so that my sister would be able to grow them successfully, too.
I've found that the potting mix makes all the difference - if light and fertilizer are equal.

The taller the plant grows, the more likely it is to discard its lower leaves....
and that's why I prune the main stem and pluck any unsightly leaves.
A bare Avocado stem will resprout leaves as long as the roots are healthy.


Josh

My mix doesn't have any "soil," which is what allows it to drain freely....
plus, you can water it without worry, and you can flush it thoroughly when the
salts (from tap-water, fertilizer, et cetera) begin to accumulate.
With a porous mix, you can eliminate most of the browned leaf-tips.


Josh

Josh, that's a beautiful medium you're holding in that photo... extremely similar to what I mix for my plants.