It seems that the requirements for settled status are set up so as to allow anyone who has a right of permanent residence to qualify. That means that the five-year period does not need to be relative to any particular date.
I am not familiar enough with the law from the 1990s to have an opinion about exactly when you acquired permanent residence status in the UK, but would certainly have had it as of 2006 (when the regulations implementing the 2004 directive went into effect) if you had been working for a continuous period of five years before that. (Your time as a student might not count unless you had comprehensive sickness insurance; this requirement does not exist for a worker.)
Once you have permanent residence, you only have to meet the requirements to retain it, which you have. The current regulations, issued in 2016, would govern your absence beginning in 2017. They say, at 15(3):
(3) The right of permanent residence under this regulation is lost through absence from the United Kingdom for a period exceeding two years.
Absent other considerations such as considerations of public safety, you have a right of permanent residence in the UK as long as you return before next month.
I infer from your question that you have never received a permanent residence document from the UK. If you have, you only need to show the document and that your absence does not exceed the maximum allowed time outside the UK. If you have not, you will need to show evidence to support your claim of a continuous period of five years. It will probably be easiest to use the period from January 2012 through January 2017, since that will remove any doubt about long absences between the end of the five-year period and the start of your current absence.
From the settled and pre-settled status pages, they offer a more detailed look at the meaning of "five years' continuous residence":
Five years’ continuous residence means that for 5 years in a row you’ve been in the UK for at least 6 months in any 12 month period, except for:
- one period of up to 12 months for an important reason (for example, childbirth, serious illness, study, vocational training or an overseas work posting)
- compulsory military service of any length
Note that this applies to any 12-month period, for 5 years in a row. While that seems a bit imprecise, it certainly implies that they're specifically not talking about the five years immediately preceding the date of your application, the end of the transition period, or any other specific date.
You can also look at Appendix EU of the Immigration Rules. At paragraph 11, it seems that you qualify under condition 3:
(a) The applicant is (i) a relevant EU citizen, (ii) a family member of a relevant EU citizen or (iii) a family member who has retained the right of residence by virtue of a relationship with a relevant EU citizen; and
(b) The applicant has completed a continuous qualifying period of five years in any (or any combination) of those categories; and
(c) Since then no supervening event has occurred
Again, they don't put any conditions on the qualifying period. But let's check the definitions to be sure:
continuous qualifying period: a period of residence in the UK:
(a) which began before the specified date (or after that date where the person is a family member of a relevant EU citizen and was resident outside the UK at that date); and
(b) during which none of the following occurred:
(i) absence(s) from the UK which exceeded a total of six months in any 12-month period, except for:
(aa) a single period of absence which did not exceed 12 months and was for an important reason (such as pregnancy, childbirth, serious illness, study, vocational training or an overseas posting); or
(bb) any period of absence on compulsory military service; or
(ii) the person served or is serving a sentence of imprisonment of any length in the UK, unless they have resided in the UK continuously for at least 10 years (and have the right of permanent residence in the UK under regulation 15 of the EEA Regulations) and the decision-maker considers that:
(aa) before serving a sentence of imprisonment, the person had forged integrating links with the UK; and
(bb) the effect of the sentence of imprisonment was not such as to break those integrating links; and
(cc) taking into account an overall assessment of the person’s situation, it would not be appropriate to treat the period of imprisonment as breaking the continuous qualifying period; or
(iii) any of the following, unless it has been set aside or no longer has effect in respect of the person:
(aa) any decision or order to exclude or remove under regulation 23 or 32 of the EEA Regulations; or
(bb) a decision to which regulation 15(4) of the EEA Regulations otherwise refers, unless that decision arose from a previous decision under regulation 24(1); or
(cc) an exclusion decision; or
(dd) a deportation order, other than under the EEA Regulations; and
(c) (where the period is less than five years) which continues at the date of application
It's a lot to wade through, but they only mention the date of application if the continuous period is less than five years in length. So, once again, you seem to be fine here.
I don't believe I've ever encountered the word "supervening" before, but fortunately the drafters of the appendix have anticipated that, for they also define "supervening event":
supervening event: at the date of application:
(a) the applicant has been absent from the UK for a period of more than five consecutive years (at any point since they last acquired the right of permanent residence in the UK under regulation 15 of the EEA Regulations, or since they last completed a continuous qualifying period of five years); or
(b) any of the following events has occurred, unless it has been set aside or no longer has effect in respect of the person:
(i) any decision or order to exclude or remove under regulation 23 or 32 of the EEA Regulations; or
(ii) a decision to which regulation 15(4) of the EEA Regulations otherwise refers, unless that decision arose from a previous decision under regulation 24(1); or
(iii) an exclusion decision; or
(iv) a deportation order, other than under the EEA Regulations
So it looks like they're even being rather more generous than the permanent residence criteria specified in the 2016 regulations, and that an absence of less than five years preserves your eligibility for settled status. If I were you, though, I would suspect that to be a mistake, and make a point of getting back to the UK within the two-year period specified in the regulations.