Serums contain much higher concentrations of actives (i.e., vitamins, peptides, hyaluronic acid, etc.) than moisturisers.
Moisturisers provide much needed hydration but as a rule are much thicker than serums, therefore cannot penetrate deep into the skin.
Serums are typically lightweight
Serums are typically lightweight, non-occlusive and will readily penetrate deep into the skin, delivering, super-concentrated actives to the targeted site in the skin. Serums are results-driven and formulated to address specific skin conditions such as ageing skin, hydrations, pigmentation, congestion / breakouts, etc.
Serums have been around for some time but with new peptides and more stable vitamins becoming available, plus the more recent encapsulation technology, serums are fast becoming a true powerhouse capable of creating real changes deep into the skin.
Think about your main skin concern when choosing a serum. When applying your serum always apply to clean damp skin after you have double cleansed, exfoliated, toned and follow with moisturiser morning and evening. Serums can be water-based or oil-based. If you are using more than one serum always apply the serum in order of the thickness starting with the thinnest and finishing with the thickest.
There is no set age to start using a serum, as usage generally depends on the skin concern. Serums are an investment with a long-term gain. Invest and you will reap the rewards.