What are these on a young Avocado tree' should I leave them or remove them

This is new growth on your avocado tree.  The leaves/branches with the black stems are likely to fall off.

It is normal for tropical plants to lose some or all of their leaves in cool weather.  Too much water can also cause them to drop their leaves.

Always check the soil around the tree before watering.  You can do this by inserting your index finger into the soil to the second knuckle. If it's dry, water. If it's moist, then leave watering for a few more days. 

To encourage plenty of healthy new growth and assist with fruit production, try 'Yates Thrive Citrus & Fruit Natural Fish & Seaweed+' as a complete plant food, specially developed for boosting healthy grown and yield of juicy fruit in citrus and other fruiting plants. 

Click here for more info on this product: https://www.yates.com.au/product/garden-fertilisers/organic-based/yates-thrive-natural-citrus-fruit-concentrate 

Avocado trees are prone to a fungal issue called anthracnose.  This can be treated with Liquid Copper Fungicide, a broad-spectrum fungicide for control of a wide range of diseases on fruit, vegetables and ornamentals.

Click here for more info on this product: https://www.yates.com.au/product/garden-disease-control/yates-liquid-copper 

Avocado trees can take a number of years to grow fruit.  Grafted trees from the nursery can take up to 5 years to produce fruit, but if planted as a seed in the garden, they can take up to 10 years!

Be patient and you will be enjoying your own smashed avo on toast