Does a walnut tree ever become to old to bear fruit.
Age is not the only factor influencing tree growth and reproduction.
All trees must grow every year to continue their lives. In fact the bulk of the living tree is actually only the outermost rings, the thin layer of cambium just under the bark, and the buds. The rest of the tree, the woody rings inside the trunk and the outer bark, are all either dead or mostly dead with only a few living cells scattered around.
The longer the tree lives the more tree has to be supported and supplied. Aging and unproductive trees (in the case of your walnut) are often suffering from a lack of resources (water, mineral nutriment from the soil, of if they are shaded, sunlight) than mere age.
Also, older trees will have built up a few problems. Trees do not heal as we do. The bulk of their living tissues are a thin sheet just beneath the bark with nearly every other part of themselves being an accumulation of wood. Instead, the tree surrounds and closes off the damaged parts of itself and overgrows them. So really any damage done to a tree is permanent and merely sequestered away. Old trees will have accumulated lots of damage over the years of their lives. These damaged areas represent blockages in the tree's systems that have to be worked around. Unlike a very young and undamaged tree, an old one will have few direct paths for water and sap to move through the tree. Pockets of decay will exist that the tree will be on constant defense for (costing the tree extra effort), any weakness is also likely to invite insect predation creating more damage and weakness for the tree costing it still more effort. The point of this long paragraph is that an older walnut will have likely endured and is enduring many stresses that can interfere with production of walnuts.
Sometimes you can assist a producing tree by insuring it has resources like water and fertilization, but, big but, when you start providing for a tree it increases its growth year over year. When you stop providing the tree will begin to decline due to a lack of resources.
Summary of major points
- old trees are often less vigorous and productive because they are unable to acquire the resources to support their ever enlarging selves.
- old trees are often less vigorous and productive because they have accumulated problems (damage, disease, pests) that require the tree expend its resources on to just stay alive, preventing it from being more productive.
Hope this helps you out a bit. Tree aging and tree decline are interesting topics that still have lots of work to be done on them.