The modelling industry is constantly changing.  Models can come in all shapes and sizes for commercial print.   Advertisers look for people/models that best represent their product that they are selling.   There are different areas of modelling - Fashion Models, Commercial Print Models, Hand Models, Beauty Models, Character type models.

When you see an ad, look at the ad, see what the advertiser is selling and that will answer your questions on how old, how tall, do you have to be to model.

If we are talking Fashion and Clothing there are height and size requirements.   If an advertiser is showing a new power tool, perhaps a gentleman ( Dad type) would be used to model in the ad.

You are never too old to get started in this industry.   Models today must have a great personality.  A client will book a model because of looks and personality.  Personality is the key to success in any business.  So put on your smiles, have a great attitude and get started, what have you got to loose!