My first plums and peaches are often as good or better as following crops. Some varieties of apples are known to need to slow down before quality goes up, producing over-sized low flavor fruit at first- prone to rots and other problems. This is often the case with Honeycrisp apple.

Sometimes juvenility leads to a more vegetative plant that has the same problem as if too much nitrogen were added- big watery fruit.

Other times the results are the opposite, where a young tree (first crop especially) only produces a few fruit and the sugar is concentrated in them, basically behaving like an extremely well thinned tree.

I'm interested in that question, but more on the plant science for reasons "why". Sorry do not have the answer.

Many of my citrus fruits do not taste as well in their first fruiting, but this could be for reasons other than just "juvenile citrus don't produce as well".

I can say with more certainty that my Eureka Lemons are definitely tasting much better the older the tree. But again, why is that? is the real question. Perhaps the nutrient to fruit ratio is better, a consequence of root establishment, surrounding soil improvements made in the last 10 years, or what not.

I have seldom noticed much difference. My early fruit is usually high quality. But it is easy to imagine just what hman mentioned. A young tree can be overly vigorous. That means it has a lot of water and nitrogen relative to it's size. Too much water and too much nitrogen can really ruin fruit quality.

In my experience, this is the most important lesson a fruit grower can learn if they want high quality fruit.

I suspect the science would be that the cells in quicker growing trees are larger and the fruit is larger because of the size of its cells and most of this size is water. Young trees tend definately to get more vegetative growth relative to their size than older trees.

Not a complete explanation and just a theory, but I'm not a botanist. I've read that this is what happens when vegetables are fed too much quick release N- their weight consists of relatively more water and as a result they don't have as much taste, nutrient content or sugar.

oranges have the reputation that they are better as time goes on. I think in general, the first year or 2 for most trees can be stressful, and if stressed, the tree will not have as quality fruit. One of the things that makes an old tree old is that it has been getting good care and is getting what it likes, so, it only stands to reason that a healthy an old tree will have high quality fruit, but so will most young trees if they are perfectly cared for.

When it comes to figs fruit tends to be better after after a few years.