From soviet times Russia have a good mobilization system. Every town district have its own mobilization office (for example, Moscow with 27 districts has 27 of them). Usually, they have 1-2 officers and 4-5 civilians working there. They are divided on officers and low ranks departments.
Each Russian citizen (all man and all woman with military education) should make registration in local mobilization office and inform it when they move somewhere, change official job and etc. Also, you need to register in mobilization office to take pension if you spend in Army 20+ years.
Surely, many people after they spend a year in the army, or 3 years in military additional courses in university, or retired from the army forget about mobilization offices, but they get all documents about each person from Ministry of Defense (After you finish your service in education they inform your local mobilization office, because each Russian citizen have official place of residence).
All information about people is systemized. Each district office knows military occupation and personal experience, they have documents about person. So, it is very easy to mobilize people they need.
In Russia many officers serve for 20 or 25 years and take a good military pension, its twice bigger then moderate civilian (also they mast buy you house or apartments). In these 20-25 years are included 5 years in military high school! Also, if you take part in active military companies or serve in the High North, they count your 1 year of service as 2 or even 3. So, you CAN finish your service in 38-43 years, or even in 35, or even earlier if you are a special force officer who served in Syria or Northern territories (If you`ll try to become a general you can serve up to 60 years (senior officers major, -colonel-lieutenant and colonel). Surely usually such people are officers or warrant-officers. Many of them after retirement work in different security services (banks, markets, so on). Also, it`s important to know – all guys who retired from the Army have big files about their career in mobilization offices, they are in contact with them because of pension and so far!!!
So when you start mobilization you can easily find in your district all officers and warrant officers and some sergeants of military specialties you need. Surely, usually they be after 40 y.o! So mostly all this part of mobilized people is 35+. They are very patriotic, some of them were born in USSR and they are motivated to fight. Also their nowadays civilian salaries usually are less, then money they will be payee when they are back in Army!
All people who were conscripted in Russian army as privates (in 18) spend their 2 or (after 2008) 1 year. All they studied at list 3 month to take military specialization. Many of them after 1 or 2 years of compulsory service stayed in Army for 2-3 years to get some money and benefits for education. All they are in contacts with military offices and they are mobilized now too!
So, for lower ranks the best variants for Russian mobilization are people of 25-30 (they don’t forget what they studied in the Army) who spend in the Army a year or more. Russian army pay very good salary for mobilized privates, corporals and sergeants also this people don’t sign a contract for 1-2-3 years. Also, Russian army take a lot of older people because they are much more disciplined and motivated, served in army for 2 years (so served before 2008 and are at least 32 now).
Also, it is important to know. Mobilization order can be handed over only in official residence or working place of the person. They can’t stop people in the street(like in Ukraine), it’s not the total mobilization in Russia! Older people usually have official jobs, they have families and are to buy apartments so it’s easier to mobilize them.
Also, mobilization offices chiefs have a plan of mobilization and they should take exact number of people they were ordered (different for each district). So, they will start to mobilize people they can find. Because of that real age of mobilized of lower rank is about 30. And it’s at least 15% of people who are older than 40. Many of them came himself!
What is important to emphasize is that 60-70% of mobilized people will never reach front zone. And they understand it. They will be sent in new reserve regiments that will change regular troops somewhere in Estonian or Chinese border, and regular troops from this place will be send to frontline. Though if we`ll get second wave of mobilization they will be already more or less retrained and can be send to frontline too. Other 40-30% are more or less professionals. Some of them will be replenishment of acting army, and some of them will be formed in new regiments.