Yes it's applicable to you

If the upper age limit for the general category candidate is 40 years alone, then you may not be eligible for applying for this post, even though you are already an employee of a Govt. PSU. 

If there is any specific provision  is available for relaxing the upper age limit as mentioned in the advertisement, then you may be eligible, if not then perhaps your application may not be entertained.  

limit is relaxable for Central/U.T. Govt. Servants up to 5 years as per instructions issued by the Govt. of India from time to time. (10 years for persons belonging to Scheduled Castes/Scheduled Tribes and 8 years for persons belonging to other Backward Classes in respect of the posts reserved for them) in accordance with the instructions or orders issued by the Government of India. A candidate claiming to belong to the category of Central Government servant and thus seeking age relaxation under this para would be required to produce a Certificate in the prescribed proforma issued after the date of advertisement from his/her Employer on the Office letter head
In case of JCOs/ORs and equivalent of the Navy and Air Forces: By various Regimental Record Offices.
Naval Records, Bombay

to the effect that he/she is a regularly appointed Central Government Servant and not on casual/adhoc/daily wages/hourly paid/contract basis employee.
The age relaxation will be admissible to such of the Government servants as are working in posts which are in the same line or allied cadres and where a relationship could be established that the service already rendered in a particular post will be useful for the efficient discharge of the duties of the post(s) recruitment to which has been advertised. Decision in this regard will rest with the CommissiOn 

You would not be-entitled to age relaxation as you are not central government or state govt servant but PSU employee 

It appears that you have not went through the contents of the employment advertisement thoroughly.

If it contains the relaxation then you can be sure of it, if it is silent about your query then you can confirm that you are not eligible.

You need to take a phone consultation and give me all the details

There is no order by Central Government granting such general relaxation. If there is any such relaxation, it will be mentioned in the notification. Check in the notification for answer to your question. Every appointment has its specific requirements. No general Rule is applicable.

- As per rule , employees of a public sector undertaking (PSU) are not government servants, and only the  employees working directly under the central government or any of the state governments or the Government of a union territory are considered as government servant . 

- Since, you are working in PSU , then it not applied in your case for getting age relaxation. 

Dear Client,

                 UPSC Civil Services Exam Age Limit: Candidate must be a minimum of 21 years of age and must not be more than 32 years of age. General Category & EWS: 32 years; 6 attempts.Therefore, even if age relaxation is applicable still you will not reach the age of 32 as you are 42 but since the advertisement states that it is applicable till 40 years you should go ahead and take a chance and also firstly verify with an good legal practitioner if the particular job advertisement is indeed a genuine one.

Thanks & Regards