To what age does the average tree live? For instance, how long could I expect a tree 6-8 m (20-26 ft) tall to survive? And at what age would it be mature enough to flower?
Small trees of the size you mention vary greatly in the age at which they start to flower; it may be about 5 years, or it may be 10 or more; and they live for 40-50 or more years. The larger forest trees, such as the tulip tree (Liriodendron tulipifera), oak (Quercus robur) and beech (Fagus sylvatica), may live for hundreds of years, but will be slow in starting to flower— perhaps not for 20 years.
A horse chestnut in my neighbour’s garden has developed curious, brownish plate-like growths sticking out of the trunk, not very far above the ground. What are they caused by?
These hard woody growths are the fruiting bodies of a fungus which has infected the tree internally. They produce millions of spores which can infect other trees, so they ought be sawn off and burnt There is no cure for the infected tree, which will eventually die; cut it down and destroy it.
Can I make use of the piles of leaves which accumulate from trees every autumn?
Yes, they can be used to form humus-supplying material if you gather them into heaps contained in wire-netting or compost bins, and allow them to rot for about a year. Restrict the heap to leaves: do not include twigs or bark, which take many years to decay.
A young ash tree in my garden appeared to be growing well until this spring, when the leaves on a couple of branches wilted for no apparent reason. A close look showed holes in the bark, and what looks like sawdust around them. Is there some beetle at work internally?
From your description it sounds as though your ash tree has been infested with caterpillars of the leopard moth. They feed on the wood for about three years; they grow about 100 mm (4 in) long and are cream-coloured with brown heads. Bad attacks by these caterpillars can kill a young tree. Fill the holes with HCH (BHC) solution and seal them with putty; or, if only one or two branches are affected, cut these off and bum them.