I just read this article which was quite informative: http://www.veganhealth.org/articles/omega3
To sum up, the ALA you get from 1,5-2 teaspoons of flaxseed oil per day is converted to sufficient levels of EPA and DHA provided the intake of omega-6 is low. This amount of ALA could also protect against heart disease. But, in a few studies a high intake of ALA was associated with a higher prevalence of age-related macular degeneration (AMD) and higher lens opacity, which can lead to cataracts. The subjects were omnivores and one can suspect that some of that ALA had been heated. Whether unheated ALA could have a harmful effect on vegans is unknown (and will probably remain unknown since the effects show at an older age and there aren’t a lot of studies being made on old vegans). Therefore, the author suggests we take algae capsules for EPA and DHA and a smaller amount of flaxseed oil for the ALA.
In the said studies, the increase in risk of eye disease was about 50-100%. I looked at some prevalence statistics and it seems they are quite rare before the age of 65, but increase dramatically after 70 so that roughly 30% of the “old-old” are affected. If the risk was raised with 50-100% it could mean that around 45-60% would be affected. Cataract can be treated, but for AMD there isn't really any cure.
Would this make you adjust your intake of omega-3? Does anyone interpret the article differently than I did?