Mostly, I cannot recommend taking probiotics very regularly, unless you have a certain issue (eg chronic constipation) and you’ve discovered through experimentation that certain validated probiotics (Eg Culturelle) help resolve them.
BioGaia (Lactobacillus reuteri DSM 17938) is the one exception to this rule. For reasons elaborated in this blog post, I’ve decided after extensive reading that taking this probiotic continuously makes a lot of sense, and is worth the small expenditure ($26.99 for a 2-month supply) to anyone with a disposable income. My daughter had it daily since around 7 days of birth, all the way to maybe 2.5 years. Now, the frequency is slowed to every 2-3 days. I take it myself and recommend it to older family members.
For anyone who wants to read more on BioGaia, this is a good resource, as is its Wikipedia entry.
If you have a breastmilk-fed baby with colic or a baby born of C-section, BioGaia is no doubt the best probiotic choice.
The infant form (10 mL, a 2-month supply for $26.99) is its most affordable form. You get the same in chewable forms for older kids/adults, but they are nearly double the price. I hence recommend everyone take the infant version, as the dose per day is the same in all forms and does not vary for older children or adults.