Types of brake fluids?

There are several forms that meet the requirements of the classification of the US Department of Transportation: DOT 3, DOT 4, DOT 5.1, and separately from them DOT 5 and DOT 5.1 / ABS, which differ radically from other categories by composition. FMVSS 116 also requires that DOT 5 fluids should be dark red, while DOT 3, DOT 4 and DOT 5.1 should be from colorless to amber.
What is the term of the brake fluid replacement?
It is recommended to replace the brake fluid every 1.5–2 years. Exceeding the recommended shelf life of the brake fluid has an adverse impact on the brake system efficiency down to its failure.
Could brake fluids be mixed?
It is allowed to mix different brake fluids, but provided that they are glycol based. If the base is different or there is no indication on the label, mixing is not recommended. Adverse consequences may include stratification, turbidity, precipitation, and unbalanced additive set. It is recommended to replace completely the brake fluid.