at the age of three months.

Der Satz ist, wie er da steht, mit "three months of age" nicht korrekt.

"At three months of age the mice express ......." - so würde es gehen.

I think either of yours is fine, not wrong at all. I don't see any difference between them except that 'three months of age' sounds more literary, which would not be the best style for a scientific paper. In fact, you can simply say 'at three months' and it will usually be clear enough without the word 'age,' unless some other interpretation is possible, like after three months on Drug D (but then you would usually use 'after,' not 'at').

Werner's suggestion is a little better, though, because it puts the time first and saves the symptoms, the most important news item, for the final position, which is the strongest place in the sentence.

Support hm--us at three months etc.
This is the way it usually appears in scientific writing.

Danke schon mal für die Antworten. Beim Lesen derer ist mir aufgefallen, dass meine Vorschläge gar nicht eindeutig genug waren. Die Tiere exprimieren X schon von Geburt an (das geht aus dem vorangestellten Text schon hervor), die Y-Krankheit entwickeln sie aber erst mit 3 Monaten

Also: The mice express high levels of X, and at three months (of age) they develop y-disease.
Richtig so?

tomtom(UK) wrote: Support hm--us at three months etc.
This is the way it usually appears in scientific writing.

I haven't read enough medical/scientific papers to be sure, but I think that "at three months" is ambiguous. It could easily mean three months into the experiment, not at three months of age. I agree with hm--us's caution here.

If tomtom(UK) is sure that "at three months" always means at the age of three month's, then I guess that's OK. But to me, it's potentially ambiguous, and of course in general, it's better to avoid ambiguity.

I don't think it's likely to be too ambiguous if it's clear that they have been born recently, such as

The mice exhibit high levels of X from birth. At three months, they develop symptoms of Y.

In diesem Fall stimme ich hm -- us zu. Da im Text nie von irgendwelchen Behandlungen die Rede ist, sind die von hm -- us vorgeschlagenen Sätze eindeutig genug.

Nochmal Danke für die Hilfe!