People come to reenactment from different paths and ways. It is often said, that you need to be deeply into the topic theoretically, before you jump into collecting or reenacting. I do not agree with such approach. In most cases you have to be a grumpy old man like myself to be deeply involved in some minor historical event. And well, you aren't gonna look well in private's uniform in mid 40s.
Typical age of people in the Soviet Army
Absolute majority of people would start (and finish) their army service as privates. Everyone between the age of 18 and 27 was eligible for compulsory military service, and most of people would actually be conscripted between the age of 18 and 20. Those who were in the university would usually be conscripted after graduation (at the age of 21-22), yet there were exceptions. Also, many universities had university officer training courses, after the completion of which, the cadet would receive the rank of a lieutenant and would serve his compulsory years as an officer.
Sergeants were the same age as conscripts, as, essentially, they were just conscripts who were selected (mostly at random) to get educated in sergeants' school. However, the chance to get into such school would be higher if you were either older than 20 or had at least some years on university. That was not a rule, but general practice.
If person would choose military career for himself, he would usually start his education in a military university at the age of 16-17. This means, that by the time he graduated he would be around 20-22 years old (depending on the course). The military career in the Soviet Army was quite straight forwards, one could basically go all the way up to lieutenant-colonel without much effort, just doing his job without any major screw ups. The promotions would come every couple of years.
Here is the typical age for different ranks:
Private - 18-22 (but could be up to 29 if person was conscripted at the age of 27)
Sergeant - 20-25 (but could be any age between 18 and 29)
Warrant Officer - 25-45 (youngest possible is 21, oldest possible - 60)
Lieutenant - 21-23
Senior lieutenant - 23-26
Captain - 26-30
Major - 30-35
Lt. Colonel - 35+
The situation would get more complex after the rank of Lt. Colonel. Maybe there will be an article on that later. Also note, there were exceptions. For example, an officer could still be a captain at his 40s. This would be highly unusual, but advanced alcoholism would help to achieve this. But overall, most serviceman would fit into the table above.
Thoughts and advice on appropriate rank for reenactment at different age
So, my first advise is to choose age-appropriate impression. Soviet Army was (almost) purely conscript based. Which means, that between the age of 18 and 27 one would be called up for compulsory military service. While in the army, such person would usually hold a rank of a private, yet some would be sent to Sergeants' School and be promoted correspondingly. So, I would say, that a modern person would usually look like a Soviet conscript until the age of 30 or so. If you look at the photos, you will see that on average people looked older at the time.
If you are older than that, you have two options. You should either choose the rank of an officer or a warrant officer. Reenacting a regular officer is tricky. First of all, if you are above 30 years old, you would be at least a captain by now if not a major. In practice it would mean that you were to lead at least a company, maybe even a battalion. And even though a regular Soviet company in Afghanistan was far less than 100 people (link to a future article will be placed here), it is still unlikely that you will have a chance to take a command of such big group of people.
For that reason, you would be better off with a rank of a warrant officer. Soviet WOs were quite different from their NATO counterparts. Basically, if you decided to stay in the army after you would finish your regular two years of compulsory military service, you would be proposed to be sent to WO School (Praporshik school). So, one could easily become a WO in his very early 20s. And in absolute majority of cases, this would be the end of the career path. Well of course, after some time one would be promoted to Senior WO, but he would usually have exactly the same responsibility as before.
So, the rule of thumb is:
If you are younger than 30, choose any rank between private and senior sergeant (preferably based on your experience)
If you are older than 30, go for a warrant officer
If you are in charge of a team of 10 or more people - consider an officer rank
Summary
All of the above is just research and guidelines. Not rules. So if you are doing some airsoft LARPing, just enjoy yourself and put on generals epaulettes if you want to. However, if you are trying to put some serious work together, a movie for example, it is strongly advised to follow these guidelines.