I can only comment on this for people wishing to do a PhD in the sciences (I am not sure about other areas). If you have no lab experience, then it will certainly be difficult, if you are currently working in a research environment then things should be easier. Applying for such a job, in a university may be a good first step if you have been away from science for a number of years.
As with any change of career you will at times run into discrimination. Prejudice about employing older post-docs is sometimes more understandable. Some university pay grades make it expensive to employ older post-docs, giving the supervisor a choice between a 25 year old post-doc on a 3 year contract or a 35 year old for under 2.5 years (there may well be ways around these problems, but you will need to find somebody who is prepared to take them on). After your first or second post-doc you may be too old to qualify for many of the highly prized research fellowships. You should speak to people in your specific subject area about career prospects.