Any serious acoustic study should include gender as a variable . Men and women, due to their anatomical and physiological  condition, produce sounds with different features. The age of your study depends on what you want to analyze . For example, you'd want to analyze the generation of adults between 40 and 50 who were born between 1966 and 1976. Read about the sociolinguistic variables can help you.

Thanks a lot dear Maryluz.

Actually, I want to describe the vowel system of a language that has not been acoustically analyzed before. Would it be possible to take ONE generation and claim that this is the vowel system of that language?  If it is possible, which generation to take? It is not an easy task to take various age groups and compare F1 & F2 means to come up with the vowel chart of that language, though it may the most accurate way.

Ammar: For methodological tips on informant choice, as well as sampling and analysis of previously unrecorded languages, the Handbook of the IPA may be a good place to start:

https://www.internationalphoneticassociation.org/content/handbook-ipa

The Journal of the IPA regularly publishes phonetic illustrations of previously unrecorded languages (vowels, consonants, prosody), and is dedicated to research on the phonetics of any language. Browsing a couple of journal issues will give you a few more tips:

https://www.internationalphoneticassociation.org/content/journal-ipa

You may even consider submitting to this journal an article on your findings about the vowel system of the language you’re working on, and/or a full illustration of its phonemic inventory? Instructions for contributions are here:

http://journals.cambridge.org/images/fileUpload/documents/IPA_illustrations_ifc.pdf

All best for your research

Madalena

Unless you have a massive database and unlimited time to work on this project your best bet is selecting a specific group, specially in what concerns gender and age range, and explaining your choice as well as the limitations of the study in the methodology. Working age females (let's say for example 20-40) could be a representative group, as according to the variationist sociolinguistics, see Labov, females tend to carry linguistic trends in a community and speech apparatus at that age range would be reasonably stable in terms of physical development.

In what concerns your final question, gender is key here as males and females will have totally different vocal apparatus. Once again, depending on your timeline, you could evaluate both genders but never mix the data indiscriminately. Plot results of both gender groups separately.

Thanks for the feedback dear Ferreira. 

As you mentioned, we cannot describe the vowel system of a spoken language without considering recordings by both genders due to the potential differences between them. Accordingly, they cannot be separated in the analysis to have two different vowel charts; one for males and another for females.  It is not my intention here to identify differences based on gender. My intention is rather to come up with the most accurate representation of the vowel system of the language. 

Thanks dear Madalena for the suggested sites, they are really great. 

I agree with the above.  Initially you might want to limit your subject pool to a certain age category.  After you have a vowel chart constructed, add other age categories.  I would also look at some of the early work used to construct the vowel chart for English.  An especially good source, which I use for my class, is Denes and Pinson, The Speech Chain.  Also, Ladfoged, P.  A Course in Phonetics.

Regards

thanks dear Jeffrey, I'll definitely have a look at these sources.