Math PhD application references from a postdoc Will a reference from a postdoc be considered less than a reference from a lecturer/professor?  This is assuming that I have done a dissertation under him/her and that the postdoc is in his/her (at least) third postdoc position.  It may be worth to note that he/she has graduated PhD for around 10 years.
Will a reference from a postdoc be considered less than a reference from a lecturer/professor?


As a general rule, yes, but it depends who the postdoc is, how strong (and effectively-written) their letter is, and what the letter is for. As an example, I am currently working with a smart undergraduate, and he got accepted to several very good graduate programs (US, mathematics) on the strength of his application, which included a reference letter from a postdoc he did a research project with last year. So such letters can certainly be effective and have value. It all depends on the context. 

The main point is that the most important factor in how seriously a reference is considered is not so much the job title of the letter writer (although that is still a factor that has some significance), but their credibility and reputation in the research community. Certainly many postdocs already have high credibility and name-recognition, and a letter from them (at least for a grad school application and to a lesser extent for a postdoc application; less so for a tenure track job application) will be taken quite seriously.