He wants to propose to a girl without telling her family his real age

Firstly:

There is no doubt that Allah, may He be exalted, has forbidden lying and has enjoined the believer to be truthful in his speech. In fact the Prophet (blessings and peace of Allah be upon him) warned the one who persists in lying that his ultimate fate will be Hell. He (blessings and peace of Allah be upon him) said: “And 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 Allah as a liar.” Narrated by al-Bukhari (5743) and Muslim (2607) from ‘Abdullah ibn Mas‘ud (may Allah be pleased with him).

Secondly:

In principle, it is the woman’s right to stipulate the characteristics that she wants in a husband, because she is the one who is concerned about this matter. But that does not mean that her guardians do not have the right to intervene and stipulate some conditions, if they think that [the woman’s choices] could lead to troubles between the spouses, or for other reasons. So the opinions of the woman’s family cannot be ignored completely. Therefore Islam gives the woman’s guardian control over the marriage contract, and she cannot give herself in marriage, because her guardian knows better than her the conditions required for the marriage to be successful, because of what he has of experience in life. What is required of the guardian is to choose the best for her, and to act in her best interests.

Her guardian may think that this large difference in age between you is not in the woman’s best interests, or he may not think that, for reasons that make him overlook this age gap.

This is something that varies from one country or community to another, and even from one woman to another, depending on their circumstances.

Moreover, it is unlikely that you will be able to conceal your age for a long time; rather it is likely that it will be found out, even if that is after some time has passed, and when it happens, it will lead to conflict and arguments. Anything that may be a cause of conflict between believers is forbidden according to Islamic teachings. Allah, may He be exalted, says (interpretation of the meaning):

{The believers are but brothers} [al-Hujurat 49:10].

And the Prophet (blessings and peace of Allah be upon him) said: “Be, O slaves of Allah, brothers.” Narrated by al-Bukhari (6064) and Muslim (2563).

Conclusion

Lying about the husband’s age is obviously a lie, and we do not know of any shar‘i justification for doing that. What you must do is refrain from lying in all your affairs, and do not begin your married life in such a way. You should tell her family the truth, then let them choose on the basis of complete insight, without any lying or deceit.

Before doing that, you should pray istikharah, asking Allah, may He be exalted, to guide you. For Allah, may He be glorified, is the One Who knows what is best for a person and decrees it for him, whilst the person has no knowledge of that. Allah, may He be exalted, says (interpretation of the meaning):

{perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not} [Al-Baqarah 2:216]

{perhaps you dislike a thing and Allah makes therein much good} [An-Nisa’ 4:19].

And Allah knows best.