Praise be to Allah.
It is not permissible to lie about one's age in order to get the free medical treatment offered by the company. This comes under the heading of lying and deceit, and the Prophet (blessings and peace of Allah be upon him) said: Whoever deceives us is not one of us. Narrated by Muslim, 101.
And he (blessings and peace of Allah be upon him) said: Beware of lying, for lying leads to wickedness and wickedness leads to Hell. A man may continue to tell lies and endeavour to tell lies, until he is recorded with Allaah as a liar. Narrated by al-Bukhaari, 6094; Muslim, 2607.
What the believer must do is be sincere in word and deed. Allah, may He be exalted, says (interpretation of the meaning): O you who believe! Fear Allah, and be with those who are true (in words and deeds) [at-Tawbah 9:119].
And Allah knows best.
Reproduced from Islam QA