It could be mites if  there are webs present on the leaves or small insects you can see with magnifying glass. These tiny insects are sucking juice from the leaves. Wipe leaves with soapy water then spray with hose. Repeat. Use am insecticide that kills mites if this fails. The other cause could be too much direct sun. Do you live in an area where it has become sunny and warm? The tree needs time to adjust to full sun just like we do. I think the older leaves look clean and mite free so my bet is on the sunburn. Snip off sad leaves and plant will adapt to sun over time. Try shading the trees with an umbrella during heat waves. Keep growing, Marianne Binetti