1) just because some grains are rounded  we cant say that rounded grains are relatively older than euhedral ones. The morphology of the grains depends upon its transportation history and provinance rock.

2) Color in no way an indicator for the relative age of the grain. It entirely depends on the trace elemental content of the mineral grain.

There is absolutely no link between grain size, morphology, color and relative age of the zircons.

The ratio of parent radioactive uranium to daughter lead atoms in zircons can be used to calculate the ages of the crystals. Zircon U-Pb dating is the most commonly used method for isotopic geochronology. Therefore, interpretation of zircon U-Pb ages can be constrained by its internal structure, trace element composition, mineral inclusion and so on. 

There is no norm about this. However, I have noticed well-rounded, inclusion free and finer-grain size associated with older zircons due to successive reworking, particularly in the Tertiary sediments subjected by rapid unroofing and transport from the source orogen.

The rounding normally is related to weathering out of individual grains from the original pluton and subsequent transport and depositional history. They are very resistant to weathering so can survive many cycles of erosion, transport and deposition. The hardness of zircon is not related significantly to colour since the latter is the result of trace impurities.However their colour variations and survival makes them very useful in heavy mineral studies for tracing the origins of sedimentary deposits.

Stuart.

 Dr. Towe: I observed extremely well-rounded zircon in the folded Tertiary sediments in Bangladesh and usually concentrated in very fine-grained sand. Obviously zircon was recycled from the older rocks under fast flowing, high gradient streams originating from the newly developed Himalayan Mountain. Transport mechanism coupled with nature of source orogen and distance of transport can be envisioned as constraining roundness in zircon.

Some relevant articles associated with zircon morphology and provenance identification:

http://rimg.geoscienceworld.org/content/53/1/469

Reviews in Mineralogy and Geochemistry

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Atlas of Zircon Textures

Fernando Corfu, John M. Hanchar, Paul W.O. Hoskin, Peter Kinny

DOI: 10.2113/0530469 Published on January 2003, First Published on January 01, 2003

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INTRODUCTION

The mineral zircon is extremely variable both in terms of external morphology and internal textures. These features reflect the geologic history of the mineral, especially the relevant episode(s) of magmatic or metamorphic crystallization (and recrystallization), strain imposed both by external forces and by internal volume expansion caused by metamictization, and chemical alteration. The paper presents a selection of both the most typical, but also of the less common, features seen in zircon, categorized according to the different geological processes responsible for their formation. The atlas is intended as a general guide for the interpretation of zircon characteristics, and of related isotopic data.

Zircon has become one of the most widely used minerals for the extraction of information on the prehistory and genesis of magmatic, metamorphic and sedimentary rocks. Much of the geological usefulness of zircon stems from its suitability as a geochronometer based on the decay of U (and Th) to Pb, but in addition it is also the major host of the radiogenic isotopic tracer Hf, and it is used to determine oxygen isotopic compositions and REE and other trace element abundances, all of which yield useful clues concerning the history of the host rock, and in some case, the parent rock in which the precursor zircon crystallized.

One of the major advantages of zircon is its ability to survive magmatic, metamorphic and erosional processes that destroy most other common minerals. Zircon-forming events tend to be preserved as distinct structural entities on a pre-existing zircon grain. Because of this ability, quite commonly zircon consists of distinct segments, each preserving a particular period of zircon-formation (or consumption). A long experience and modern instrumentation and techniques have provided the “zircon community” the means to image and interpret preserved textures, and hence to decipher the history and evolution of a rock.

https://link.springer.com/article/10.1007/BF00381441

Contributions to Mineralogy and Petrology

August 1980, Volume 73, Issue 3, pp 207–220

Pupin, J.P.

Zircon and granite petrology

http://rimg.geoscienceworld.org/content/53/1

Reviews in Mineralogy and Geochemistry

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January 2003; 53 (1)

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Structure and Chemistry of Zircon and Zircon-Group Minerals

Robert J. Finch, John M. Hanchar

Reviews in Mineralogy and Geochemistry, v. 53, i. 1, p. 1-25, Published on January 2003, First Published on January 01, 2003, doi:10.2113/0530001

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The Composition of Zircon and Igneous and Metamorphic Petrogenesis

Paul W. O. Hoskin, Urs Schaltegger

Reviews in Mineralogy and Geochemistry, v. 53, i. 1, p. 27-62, Published on January 2003, First Published on January 01, 2003, doi:10.2113/0530027

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The age of fresh, unweathered zircons by radiometeric dating will only provide the date the crystal was formed. However by studying the colour and other physical properties of a reasonably large number of zircons in a sedimentary rock such as till, the source of the sediment can often be inferred. To do this, it is essential to know the properties of the zircons that may be present in all the rocks over which the sediment may have passed.

Stuart.

I concur.