The problem with old frequencies
As mentioned previously, frequencies are regulated because the spectrum needs to be shared among different applications. Apart from military usage, frequencies are used by civilian air traffic, sea traffic, radio stations (long waves, middle waves, FM band), amateur radio among which the CB (Citizens Band) used by trucks... and RC.
Thanks to strict regulations, no plane has ever crashed due to a Barbara Streisand record played on a weired radio station. Ever. Nevertheless, one must remember that the Martians failed to invade Earth due to Slim Whitman (Mars Attacks!).
However, many RC users of the 80's experimented the loss of control of their model due to a truck with a CB crossing by because both were using the same band. In fact, the bottom of the spectrum (the lowest range frequencies) was allocated to "minor uses" (ie uses that authorities didn't care about because they were not dangerous or of commercial interest).
This is the reason with RC federations issued very strict regulations to allocate the different frequency bands available for RC usage (they still apply). Indeed, because loosing control of an RC plane was potentially more dangerous than loosing a boat in the middle of a lake, the frequencies were strictly allocated between the different RC uses. Here's the fun: due to regulation evolutions, a transmitter with a specific frequency could be first compliant with any RC use, then forbidden for everything but RC airplanes and finally compliant again for everything RC in just a matter of years. Of course, these regulations and evolutions were (and still are) country-specific.
In concrete, either you're in a club or you have no clue whether the frequency you are using is legal and if it is legal for your use in your country. This applies to every frequency in the 26-27MHz, 35MHz, 40-41MHz and 72MHz bands, either AM or FM when it applies.
What should I do with a transmitter using old frequencies?
Officially, you must comply with the precise usage allowed for the frequency you are using: a reliable hobby shop or a club should be able to provide valuable information about this. Unfortunately, it is difficult to get reliable and up to date information about this for years.
So, unofficially, just care about one thing: what's around.
- You are alone:
Check if anybody is using a transmitter nearby (100 meters around). No one? Everything should be fine.
- You are not alone:
Check the frequency used by the other person by asking him. In most cases, he will use a 2.4GHz transmitter so there will be no problem: worst case, he will not understand your question because he never knew the golden era of frequency interferences... or he did and the question will make him smile. In the rare case someone else is also using an old frequency, there is a "0.2MHz" rule. You need at least a 0.2MHz difference between frequencies of the same band to avoid any problem (for example: 26.815 and 26.835 are OK). Besides, you can use the exact same frequency when one is AM and the other is FM (no problem since the modulation is different). If you use different bands (27 and 41MHz for example), no problem either.
Even if RC airplane purists may get upset when reading this, there are so few people still using old frequencies that you will probably never experience the smallest problem. As for the beloved CBs from the 80's that were driving our models mad, most if not all of them have now disappeared thanks to mobile phones.
The new band of frequencies: 2.4GHz
This has been the standard for several years now. Everyone or so has already switched to this band, especially since the transmitter offering for classic crystal frequencies has almost disappeared, except for entry-level devices (as per 2014, you have to specifically search for them if you want to find one).
The greatest advantage of this band of frequencies is that it is secured. In fact, it works about the same as WiFi devices (the band is actually shared with WiFi): the transmitter and the receiver "discuss" using a protocol that secures the communication. To make it short and clear, a 2.4GHz transmitter will automatically scan available frequencies in the 2.4GHz band and select the best free channel to secure the communication with its receiver. The identification of the receiver is made during the binding process: this operation is generally factory performed but it is very easy to do it yourself (you need to do it to "attach" additional receivers to a transmitter). The other transmitters and receivers around (in the range) will do the same, each one selecting a free channel. In concrete, about 40 2.4GHz transmitters can work at the same time without any problem.
Apart for old transmitters fans who do accept the risk to loose the control over their model due to any possible interference, it is recommended to abandon transmitters using old frequencies and to replace them with modern 2.4GHz units. Indeed, this is a cost and a vintage model fitted with modern electronics may loose some of its authenticity, but security wise, 2.4GHz is the best solution.