•The knowledge of elderly T2D patients in Slovenia stayed unchanged 2010–2020.•The average level of knowledge has slightly worsened for 80+ patients.•Adapted educational efforts are needed in times of accessible general information. To achieve better treatment decisions, type 2 diabetes patients need to be empowered also through knowledge increase. The aim of this study was to evaluate and compare the level of knowledge and overall perceptions of type 2 diabetes within the elderly diabetic patients before and after the National Diabetes Prevention and Care Development Programme 2010–2020. Diabetes knowledge test was used in two cross-sectional studies in 2011 and 2020 where the samples of type 2 diabetes patients 65+ were surveyed. Besides descriptive statistics, non-parametric tests and general linear model were used to compare the level of knowledge. The comparison reveals that in the last decade the general knowledge about diabetes has not significantly changed (U = 16942, p = 0.809). The average scores in 2011 and 2020 were 7.98 ± 2.41 and 7.96 ± 2.36 respectively. The average level of knowledge has slightly worsened for patients in the age group 80+, while it remained approximately the same in the other three age groups (65−69, 70−74, 75−79). Our study has shown that despite the National Diabetes Prevention and Care Development Programme the knowledge of elderly diabetic patients in Slovenia remained at the same level or worsened.