Age covariate in GWAS

Question

[aaaniich]

Good afternoon! Please advise, I am reviewing association studies on type 1 diabetes and notice that many of them include age as a covariate (e.g. Tanigawa Y et al. PLoS Genet, Privé F et al. Am J Hum Genet (2022), Oram RA et al. Diabetes Care), but there are also studies without including this covariate (e.g. Aly DM et al. Nat Genet (2021), Ferrat LA et al. Nat Med (2020)). Can you explain how necessary this covariate is in the context of this disease? After all, there is no per se effect of age on type 1 diabetes.

I have a control who is very much older than the sample of diabetics and I don't know how to get it right.