First of all a useful clarification: If you have changed your address on your Citizen's Card, you do not have to revalidate your driving licence.
The dates for the compulsory revalidations, depends on the date you took it out and the type of card you have. Find, at the end of this article, the table that applies to you and check, for your type of card, the anniversaries on which you must update.
Revalidate your licence on time, otherwise a fine is imposed and, if you let more than 2 years pass after the time limit without revalidating your licence, you will have to take a driving test to get a new licence.
You should revalidate your licence up to the day before your marked birthdays. For example, if you have to revalidate your licence until you are 50, you must do so by the last day you are 49. The licence expires on the day of your 50th birthday. The deadline for dealing with revalidation starts 6 months earlier.
Do you need a certificate of aptitude?
In the table below, you can check whether you need to provide a medical certificate or a certificate of psychological fitness (CAP). If you need a CAP, you must scan it and send it when you make the request through the internet. If you need a medical certificate, the doctor himself sends it by email to the IMT.

You can request PAC from a psychologist or request the CAP from IMT.
What is the most convenient way and how to do it?
The simplest, cheapest and least risky way of revalidating a driving licence at this stage is over the interneton the website of the IMTInstitute for Mobility and Transport.
Via the internet (Consult the IMTonline support manual or send an e-mail to imtonline@imt-ip.pt)
It should register on the IMT website with your fiscal number and password from the Portal das Finanças.
How much it costs and how long it takes, in the online modality
30 or 15 euros from age 70 onwards. Through IMTonline you get a discount of 10%
If you apply for revalidation online, you will receive the new letter within 10 days.
If there is no one at the given address to receive the new letter, you can collect it from the post office within 5 days. After that, it is available at Regional or district directorate for mobility and transport in your area of residence.
Deadlines for compulsory revalidations
Important note: due to containment, for licences which expire on or after 13 March 2020, or within the 15 days immediately precedingthe validity of driving licences was extended until 31 March 2021 (D.L.n.87-A/2020, of 15 October)
- If you got your licence before 1 January 2013

- If you got your licence between 2 January 2013 and 29 July 2016

- If you got your licence on or after 30 July 2016

Given the current contingencies, it is advisable to consult the first link below in order to validate other possible future deadline extensions, in addition to the one communicated on the previous page.
Sources:
https://eportugal.gov.pt/servicos/revalidar-a-carta-de-conducao
